diff --git a/go.mod b/go.mod
index 9e38376..adecd14 100644
--- a/go.mod
+++ b/go.mod
@@ -10,6 +10,6 @@ require (
 require (
 	github.com/mattn/go-colorable v0.1.14 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
-	golang.org/x/sys v0.29.0 // indirect
-	golang.org/x/text v0.26.0 // indirect
+	golang.org/x/sys v0.39.0 // indirect
+	golang.org/x/text v0.32.0 // indirect
 )
diff --git a/go.sum b/go.sum
index 0aa4e6a..3151c8d 100644
--- a/go.sum
+++ b/go.sum
@@ -9,5 +9,9 @@ github.com/mattn/go-isatty v0.0.20/go.mod h1:W+V8PltTTMOvKvAeJH7IuucS94S2C6jfK/D
 golang.org/x/sys v0.6.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.29.0 h1:TPYlXGxvx1MGTn2GiZDhnjPA9wZzZeGKHHmKhHYvgaU=
 golang.org/x/sys v0.29.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
+golang.org/x/sys v0.39.0 h1:CvCKL8MeisomCi6qNZ+wbb0DN9E5AATixKsvNtMoMFk=
+golang.org/x/sys v0.39.0/go.mod h1:OgkHotnGiDImocRcuBABYBEXf8A9a87e/uXjp9XT3ks=
 golang.org/x/text v0.26.0 h1:P42AVeLghgTYr4+xUnTRKDMqpar+PtX7KWuNQL21L8M=
 golang.org/x/text v0.26.0/go.mod h1:QK15LZJUUQVJxhz7wXgxSy/CJaTFjd0G+YLonydOVQA=
+golang.org/x/text v0.32.0 h1:ZD01bjUt1FQ9WJ0ClOL5vxgxOI/sVCNgX1YtKwcY0mU=
+golang.org/x/text v0.32.0/go.mod h1:o/rUWzghvpD5TXrTIBuJU77MTaN0ljMWE47kxGJQ7jY=
diff --git a/solutions/0/q22/solution.go b/solutions/0/q22/solution.go
new file mode 100644
index 0000000..4389b45
--- /dev/null
+++ b/solutions/0/q22/solution.go
@@ -0,0 +1,27 @@
+package q22
+
+func gen(n, lvl, used int, buf []byte, ret []string) []string {
+	if buf == nil {
+		buf = make([]byte, 0, n*2)
+	}
+	if len(buf) == n*2 {
+		return append(ret, string(buf))
+	}
+
+	if used < n {
+		buf = append(buf, '(')
+		ret = gen(n, lvl+1, used+1, buf, ret)
+		buf = buf[:len(buf)-1]
+	}
+	if lvl > 0 {
+		buf = append(buf, ')')
+		ret = gen(n, lvl-1, used, buf, ret)
+	}
+	return ret
+}
+
+func generateParenthesis(n int) []string {
+	return gen(n, 0, 0, nil, nil)
+}
+
+var _ = generateParenthesis
diff --git a/solutions/0/q25/solution.go b/solutions/0/q25/solution.go
new file mode 100644
index 0000000..118c3a0
--- /dev/null
+++ b/solutions/0/q25/solution.go
@@ -0,0 +1,51 @@
+package q25
+
+type ListNode struct {
+	Val  int
+	Next *ListNode
+}
+
+func reverse(begin *ListNode, k int) (newHead, newTail *ListNode) {
+	tail := begin
+	for range k - 1 {
+		if tail.Next == nil {
+			return begin, tail // no enough nodes
+		}
+		tail = tail.Next
+	}
+
+	// Reverse
+	cur := begin
+	prev := tail.Next
+	for range k - 1 {
+		t := cur.Next
+		cur.Next = prev
+		prev, cur = cur, t
+	}
+	cur.Next = prev
+	return cur, begin
+}
+
+func reverseKGroup(head *ListNode, k int) *ListNode {
+	if k < 2 {
+		return head
+	}
+
+	var ret, prevTail *ListNode
+	p := head
+	for p != nil {
+		newHead, newTail := reverse(p, k)
+		if ret == nil {
+			ret = newHead
+		}
+		if prevTail != nil {
+			prevTail.Next = newHead
+		}
+		prevTail = newTail
+		p = newTail.Next
+	}
+
+	return ret
+}
+
+var _ = reverseKGroup
diff --git a/solutions/0/q42/solution.go b/solutions/0/q42/solution.go
new file mode 100644
index 0000000..ed790a1
--- /dev/null
+++ b/solutions/0/q42/solution.go
@@ -0,0 +1,36 @@
+package q42
+
+func trap(height []int) int {
+	water := 0
+	mid := 0
+
+	// From left
+	maxHeight, stash := 0, 0
+	for i, h := range height {
+		if h >= maxHeight {
+			maxHeight = h
+			water += stash
+			stash = 0
+			mid = i
+		} else {
+			stash += maxHeight - h
+		}
+	}
+
+	// From right
+	maxHeight, stash = 0, 0
+	for i := len(height) - 1; i >= mid; i-- {
+		h := height[i]
+		if h >= maxHeight {
+			maxHeight = h
+			water += stash
+			stash = 0
+		} else {
+			stash += maxHeight - h
+		}
+	}
+
+	return water
+}
+
+var _ = trap
diff --git a/solutions/0/q5/solution.go b/solutions/0/q5/solution.go
new file mode 100644
index 0000000..6228213
--- /dev/null
+++ b/solutions/0/q5/solution.go
@@ -0,0 +1,37 @@
+package q5
+
+// Note: Although it can be done in O(N) with, e.g., Manacher's algorithm, the
+// input size is small enough to brute-force.
+
+func expand(s string, l, r int) (int, int) {
+	for l > 0 && r < len(s)-1 && s[l-1] == s[r+1] {
+		l--
+		r++
+	}
+	return l, r
+}
+
+func longestPalindrome(s string) string {
+	maxLen, mL, mR := 0, 0, 0
+
+	for c := range len(s) {
+		l, r := expand(s, c, c)
+		len_ := r - l + 1
+		if len_ > maxLen {
+			maxLen, mL, mR = len_, l, r
+		}
+	}
+
+	for c := range len(s) - 1 {
+		if s[c] == s[c+1] {
+			l, r := expand(s, c, c+1)
+			len_ := r - l + 1
+			if len_ > maxLen {
+				maxLen, mL, mR = len_, l, r
+			}
+		}
+	}
+	return s[mL : mR+1]
+}
+
+var _ = longestPalindrome
diff --git a/solutions/0/q50/solution.go b/solutions/0/q50/solution.go
new file mode 100644
index 0000000..2e88b24
--- /dev/null
+++ b/solutions/0/q50/solution.go
@@ -0,0 +1,25 @@
+package q50
+
+func myPow(x float64, n int) float64 {
+	neg := n < 0
+	if neg {
+		n = -n
+	}
+
+	var ret float64 = 1
+	for n > 0 {
+		v, p := x, 1
+		for p*2 < n {
+			p *= 2
+			v *= v
+		}
+		n -= p
+		ret *= v
+	}
+	if neg {
+		return 1 / ret
+	}
+	return ret
+}
+
+var _ = myPow
diff --git a/solutions/0/q56/solution.go b/solutions/0/q56/solution.go
new file mode 100644
index 0000000..920f87a
--- /dev/null
+++ b/solutions/0/q56/solution.go
@@ -0,0 +1,20 @@
+package q56
+
+import "slices"
+
+func merge(intervals [][]int) [][]int {
+	slices.SortFunc(intervals, func(a, b []int) int { return a[0] - b[0] })
+
+	p := 0
+	for i := 1; i < len(intervals); i++ {
+		if intervals[p][1] >= intervals[i][0] {
+			intervals[p][1] = max(intervals[p][1], intervals[i][1])
+		} else {
+			p++
+			intervals[p][0], intervals[p][1] = intervals[i][0], intervals[i][1]
+		}
+	}
+	return intervals[:min(p+1, len(intervals))]
+}
+
+var _ = merge
diff --git a/solutions/0/q82/solution.go b/solutions/0/q82/solution.go
new file mode 100644
index 0000000..a40254d
--- /dev/null
+++ b/solutions/0/q82/solution.go
@@ -0,0 +1,29 @@
+package q82
+
+type ListNode struct {
+	Val  int
+	Next *ListNode
+}
+
+func deleteDuplicates(head *ListNode) *ListNode {
+	root := ListNode{}
+	last := &root
+
+	c := head
+	for c != nil {
+		if c.Next != nil && c.Next.Val == c.Val {
+			dupVal := c.Val
+			for c != nil && c.Val == dupVal {
+				c = c.Next
+			}
+			continue
+		}
+		last.Next = c
+		last = c
+		c = c.Next
+		last.Next = nil
+	}
+	return root.Next
+}
+
+var _ = deleteDuplicates
diff --git a/solutions/1/q105/solution.go b/solutions/1/q105/solution.go
new file mode 100644
index 0000000..b645da6
--- /dev/null
+++ b/solutions/1/q105/solution.go
@@ -0,0 +1,35 @@
+package q105
+
+type TreeNode struct {
+	Val   int
+	Left  *TreeNode
+	Right *TreeNode
+}
+
+func bld(preorder []int, inorder map[int]int, pL, pR, iL, iR int) *TreeNode {
+	if pL == pR {
+		return nil
+	}
+
+	val := preorder[pL]
+	p := inorder[val] // position of val in "inorder"
+
+	left := bld(preorder, inorder, pL+1, pL+1+p-iL, iL, p)
+	right := bld(preorder, inorder, pL+1+p-iL, pR, p+1, iR)
+
+	return &TreeNode{
+		Val:   val,
+		Left:  left,
+		Right: right,
+	}
+}
+
+func buildTree(preorder []int, inorder []int) *TreeNode {
+	iIdx := make(map[int]int, len(inorder))
+	for i := range inorder {
+		iIdx[inorder[i]] = i
+	}
+	return bld(preorder, iIdx, 0, len(preorder), 0, len(preorder))
+}
+
+var _ = buildTree
diff --git a/solutions/1/q114/solution.go b/solutions/1/q114/solution.go
new file mode 100644
index 0000000..b52d640
--- /dev/null
+++ b/solutions/1/q114/solution.go
@@ -0,0 +1,35 @@
+package q114
+
+type TreeNode struct {
+	Val   int
+	Left  *TreeNode
+	Right *TreeNode
+}
+
+func fl(node *TreeNode) (head, tail *TreeNode) {
+	if node == nil {
+		return nil, nil
+	}
+	head, tail = node, node
+
+	lhead, ltail := fl(node.Left)
+	rhead, rtail := fl(node.Right)
+
+	if lhead != nil {
+		node.Left = nil
+		node.Right = lhead
+		tail = ltail
+	}
+
+	if rhead != nil {
+		tail.Right = rhead
+		tail = rtail
+	}
+	return
+}
+
+func flatten(root *TreeNode) {
+	fl(root)
+}
+
+var _ = flatten
diff --git a/solutions/1/q127/solution.go b/solutions/1/q127/solution.go
new file mode 100644
index 0000000..0899383
--- /dev/null
+++ b/solutions/1/q127/solution.go
@@ -0,0 +1,63 @@
+package q127
+
+// Ideas for improvement: use a hashmap to index strings instead of an O(n^2)
+// comparison and neighbor matrix; use bidirectional BFS.
+
+func reachable(a, b string) bool {
+	diff := false
+	for i := range len(a) {
+		if a[i] != b[i] {
+			if diff {
+				return false
+			}
+			diff = true
+		}
+	}
+	return diff // exactly one different char
+}
+
+func ladderLength(beginWord string, endWord string, wordList []string) int {
+	target := -1
+	neighbors := make([][]int, len(wordList))
+
+	for i := range wordList {
+		if target == -1 && wordList[i] == endWord {
+			target = i
+		}
+		for j := i + 1; j < len(wordList); j++ {
+			if reachable(wordList[i], wordList[j]) {
+				neighbors[i] = append(neighbors[i], j)
+				neighbors[j] = append(neighbors[j], i)
+			}
+		}
+	}
+	if target == -1 {
+		return 0
+	}
+
+	seen := make([]bool, len(wordList))
+	queue := make([][2]int, 0, len(wordList)) // index, steps
+	for i := range wordList {
+		if reachable(beginWord, wordList[i]) {
+			seen[i] = true
+			queue = append(queue, [2]int{i, 2})
+		}
+	}
+	for ; len(queue) > 0; queue = queue[1:] {
+		id, step := queue[0][0], queue[0][1]
+		if id == target {
+			return step
+		}
+
+		for _, n := range neighbors[id] {
+			if !seen[n] {
+				seen[n] = true
+				queue = append(queue, [2]int{n, step + 1})
+			}
+		}
+	}
+
+	return 0
+}
+
+var _ = ladderLength
diff --git a/solutions/1/q133/solution.go b/solutions/1/q133/solution.go
new file mode 100644
index 0000000..00111a9
--- /dev/null
+++ b/solutions/1/q133/solution.go
@@ -0,0 +1,39 @@
+package q133
+
+type Node struct {
+	Val       int
+	Neighbors []*Node
+}
+
+func cloneGraph(node *Node) *Node {
+	if node == nil {
+		return nil
+	}
+	seen := map[*Node]*Node{}
+
+	queue := []*Node{node}
+	for ; len(queue) > 0; queue = queue[1:] {
+		cur := queue[0]
+		if _, ok := seen[cur]; ok {
+			continue
+		}
+		cloned := &Node{
+			Val:       cur.Val,
+			Neighbors: []*Node{},
+		}
+		seen[cur] = cloned
+		for _, n := range cur.Neighbors {
+			if nCloned, ok := seen[n]; ok {
+				// Link
+				cloned.Neighbors = append(cloned.Neighbors, nCloned)
+				nCloned.Neighbors = append(nCloned.Neighbors, cloned)
+			} else {
+				// Add to queue
+				queue = append(queue, n)
+			}
+		}
+	}
+	return seen[node]
+}
+
+var _ = cloneGraph
diff --git a/solutions/1/q137/solution.go b/solutions/1/q137/solution.go
new file mode 100644
index 0000000..6e7c50c
--- /dev/null
+++ b/solutions/1/q137/solution.go
@@ -0,0 +1,22 @@
+package q137
+
+func singleNumber(nums []int) int {
+	bitCounts := make([]uint8, 32)
+	for _, num := range nums {
+		num32 := int32(num)
+		for i := range 32 {
+			if num32|1<<i == num32 {
+				bitCounts[i] = (bitCounts[i] + 1) % 3
+			}
+		}
+	}
+	var ret int32
+	for i := range 32 {
+		if bitCounts[i] != 0 {
+			ret |= 1 << i
+		}
+	}
+	return int(ret)
+}
+
+var _ = singleNumber
diff --git a/solutions/1/q146/solution.go b/solutions/1/q146/solution.go
new file mode 100644
index 0000000..f993724
--- /dev/null
+++ b/solutions/1/q146/solution.go
@@ -0,0 +1,112 @@
+package q146
+
+type lruNode struct {
+	key, val   int
+	prev, next *lruNode
+}
+
+type LRUCache struct {
+	cap        int
+	index      map[int]*lruNode
+	head, tail *lruNode
+
+	nodes []lruNode
+	p     int
+}
+
+func Constructor(capacity int) LRUCache {
+	return LRUCache{
+		cap:   capacity,
+		index: make(map[int]*lruNode, capacity),
+		nodes: make([]lruNode, capacity),
+	}
+}
+
+func (c *LRUCache) alloc() *lruNode {
+	if c.p < len(c.nodes) {
+		c.p++
+		return &c.nodes[c.p-1]
+	}
+	return nil
+}
+
+func (c *LRUCache) touch(node *lruNode) {
+	if node.prev == nil {
+		return
+	}
+	if c.tail == node {
+		c.tail = node.prev
+	} else {
+		node.next.prev = node.prev
+	}
+	node.prev.next = node.next
+
+	node.prev = nil
+	node.next = c.head
+	c.head.prev = node
+	c.head = node
+}
+
+func (c *LRUCache) evict() *lruNode {
+	if len(c.index) < c.cap {
+		return nil
+	}
+
+	node := c.tail
+	delete(c.index, node.key)
+
+	if node.prev == nil {
+		c.head, c.tail = nil, nil
+	} else {
+		c.tail = node.prev
+		c.tail.next = nil
+	}
+	return node
+}
+
+func (c *LRUCache) Get(key int) int {
+	node := c.index[key]
+	if node == nil {
+		return -1
+	}
+	c.touch(node)
+	return node.val
+}
+
+func (c *LRUCache) Put(key int, value int) {
+	if c.cap == 0 {
+		return
+	}
+
+	node := c.index[key]
+	if node != nil {
+		node.val = value
+		c.touch(node)
+		return
+	}
+
+	node = c.alloc()
+	if node == nil {
+		node = c.evict()
+		*node = lruNode{} // clear
+	}
+
+	node.key, node.val = key, value
+	c.index[key] = node
+
+	node.next = c.head
+	if c.head != nil {
+		c.head.prev = node
+	}
+	c.head = node
+	if c.tail == nil {
+		c.tail = node
+	}
+}
+
+/**
+ * Your LRUCache object will be instantiated and called as such:
+ * obj := Constructor(capacity);
+ * param_1 := obj.Get(key);
+ * obj.Put(key,value);
+ */
diff --git a/solutions/1/q151/solution.go b/solutions/1/q151/solution.go
new file mode 100644
index 0000000..df3e29f
--- /dev/null
+++ b/solutions/1/q151/solution.go
@@ -0,0 +1,19 @@
+package q151
+
+import "strings"
+
+func reverseWords(s string) string {
+	b := strings.Builder{}
+	fields := strings.Fields(s)
+	if len(fields) > 0 {
+		b.WriteString(fields[len(fields)-1])
+	}
+	for i := len(fields) - 2; i >= 0; i-- {
+		b.WriteByte(' ')
+		b.WriteString(fields[i])
+	}
+
+	return b.String()
+}
+
+var _ = reverseWords
diff --git a/solutions/1/q155/solution.go b/solutions/1/q155/solution.go
new file mode 100644
index 0000000..043fa1d
--- /dev/null
+++ b/solutions/1/q155/solution.go
@@ -0,0 +1,44 @@
+package q155
+
+type stackElem struct {
+	val, min int
+}
+
+type MinStack struct {
+	stack []stackElem
+}
+
+func Constructor() MinStack {
+	return MinStack{}
+}
+
+func (s *MinStack) Push(val int) {
+	minVal := val
+	if len(s.stack) > 0 {
+		minVal = min(minVal, s.GetMin())
+	}
+	s.stack = append(s.stack, stackElem{
+		val: val, min: minVal,
+	})
+}
+
+func (s *MinStack) Pop() {
+	s.stack = s.stack[:len(s.stack)-1]
+}
+
+func (s *MinStack) Top() int {
+	return s.stack[len(s.stack)-1].val
+}
+
+func (s *MinStack) GetMin() int {
+	return s.stack[len(s.stack)-1].min
+}
+
+/**
+ * Your MinStack object will be instantiated and called as such:
+ * obj := Constructor();
+ * obj.Push(val);
+ * obj.Pop();
+ * param_3 := obj.Top();
+ * param_4 := obj.GetMin();
+ */
diff --git a/solutions/1/q172/solution.go b/solutions/1/q172/solution.go
new file mode 100644
index 0000000..045345e
--- /dev/null
+++ b/solutions/1/q172/solution.go
@@ -0,0 +1,21 @@
+package q172
+
+// Note: actually, counting 5 alone is enough since 2 certainly occurs more than 5.
+
+func trailingZeroes(n int) int {
+	count2, count5 := 0, 0
+	for i := 2; i <= n; i++ {
+		num := i
+		for num%2 == 0 {
+			num /= 2
+			count2++
+		}
+		for num%5 == 0 {
+			num /= 5
+			count5++
+		}
+	}
+	return min(count2, count5)
+}
+
+var _ = trailingZeroes
diff --git a/solutions/1/q198/solution.go b/solutions/1/q198/solution.go
new file mode 100644
index 0000000..c72252e
--- /dev/null
+++ b/solutions/1/q198/solution.go
@@ -0,0 +1,13 @@
+package q198
+
+func rob(nums []int) int {
+	if len(nums) > 1 {
+		nums[1] = max(nums[0], nums[1])
+	}
+	for i := 2; i < len(nums); i++ {
+		nums[i] = max(nums[i-2]+nums[i], nums[i-1])
+	}
+	return nums[len(nums)-1]
+}
+
+var _ = rob
diff --git a/solutions/11/q1161/solution.go b/solutions/11/q1161/solution.go
new file mode 100644
index 0000000..6aa3884
--- /dev/null
+++ b/solutions/11/q1161/solution.go
@@ -0,0 +1,43 @@
+package q1161
+
+import "math"
+
+type TreeNode struct {
+	Val   int
+	Left  *TreeNode
+	Right *TreeNode
+}
+
+func maxLevelSum(root *TreeNode) int {
+	type qElem struct {
+		node  *TreeNode
+		level int
+	}
+	queue := make([]qElem, 0, 256)
+	queue = append(queue, qElem{root, 1})
+
+	maxSum, maxLvl := math.MinInt, 0
+	sum, lvl := 0, 1
+	for ; len(queue) > 0; queue = queue[1:] {
+		cur := &queue[0]
+		if cur.level != lvl {
+			if sum > maxSum {
+				maxSum, maxLvl = sum, lvl
+			}
+			sum, lvl = 0, cur.level
+		}
+		sum += cur.node.Val
+		if cur.node.Left != nil {
+			queue = append(queue, qElem{cur.node.Left, cur.level + 1})
+		}
+		if cur.node.Right != nil {
+			queue = append(queue, qElem{cur.node.Right, cur.level + 1})
+		}
+	}
+	if sum > maxSum {
+		return lvl
+	}
+	return maxLvl
+}
+
+var _ = maxLevelSum
diff --git a/solutions/13/q1339/solution.go b/solutions/13/q1339/solution.go
new file mode 100644
index 0000000..627f178
--- /dev/null
+++ b/solutions/13/q1339/solution.go
@@ -0,0 +1,45 @@
+package q1339
+
+type TreeNode struct {
+	Val   int
+	Left  *TreeNode
+	Right *TreeNode
+}
+
+func sumSumTrees(node *TreeNode) int {
+	if node == nil {
+		return 0
+	}
+	node.Val += sumSumTrees(node.Left) + sumSumTrees(node.Right)
+	return node.Val
+}
+
+func findNearest(node *TreeNode, target int) int {
+	if node == nil {
+		return -1
+	}
+	ret := node.Val
+	l, r := findNearest(node.Left, target), findNearest(node.Right, target)
+	if abs(l-target) < abs(ret-target) {
+		ret = l
+	}
+	if abs(r-target) < abs(ret-target) {
+		ret = r
+	}
+	return ret
+}
+
+func abs(n int) int {
+	if n < 0 {
+		return -n
+	}
+	return n
+}
+
+func maxProduct(root *TreeNode) int {
+	totalSum := sumSumTrees(root)
+	nearest := findNearest(root, totalSum/2)
+	return nearest * (totalSum - nearest) % (1e9 + 7)
+}
+
+var _ = maxProduct
diff --git a/solutions/19/q1975/solution.go b/solutions/19/q1975/solution.go
new file mode 100644
index 0000000..0c576c7
--- /dev/null
+++ b/solutions/19/q1975/solution.go
@@ -0,0 +1,40 @@
+package q1975
+
+import "math"
+
+func abs(i int) int {
+	if i < 0 {
+		return -i
+	}
+	return i
+}
+
+func maxMatrixSum(matrix [][]int) int64 {
+	var sum int64
+	min_ := math.MaxInt
+	hasZero := false
+	nNeg := 0
+
+	for i := range matrix {
+		for _, num := range matrix[i] {
+			if num < 0 {
+				nNeg++
+			}
+			if num == 0 {
+				hasZero = true
+			}
+			num = abs(num)
+			sum += int64(num)
+			if num < min_ {
+				min_ = num
+			}
+		}
+	}
+
+	if hasZero || nNeg%2 == 0 {
+		return sum
+	}
+	return sum - 2*int64(min_)
+}
+
+var _ = maxMatrixSum
diff --git a/solutions/2/q207/solution.go b/solutions/2/q207/solution.go
new file mode 100644
index 0000000..a7a223e
--- /dev/null
+++ b/solutions/2/q207/solution.go
@@ -0,0 +1,37 @@
+package q207
+
+func canFinish(numCourses int, prerequisites [][]int) bool {
+	nDeps := make([]int, numCourses)
+	revDeps := make([][]int, numCourses)
+
+	for i := range prerequisites {
+		course, prereq := prerequisites[i][0], prerequisites[i][1]
+		nDeps[course]++
+		revDeps[prereq] = append(revDeps[prereq], course)
+	}
+
+	queue := []int{}
+	for i := range numCourses {
+		if nDeps[i] == 0 && len(revDeps) > 0 {
+			queue = append(queue, i)
+		}
+	}
+	for ; len(queue) > 0; queue = queue[1:] {
+		course := queue[0]
+		for _, r := range revDeps[course] {
+			nDeps[r]--
+			if nDeps[r] == 0 {
+				queue = append(queue, r)
+			}
+		}
+	}
+
+	for _, n := range nDeps {
+		if n > 0 {
+			return false
+		}
+	}
+	return true
+}
+
+var _ = canFinish
diff --git a/solutions/2/q212/solution.go b/solutions/2/q212/solution.go
new file mode 100644
index 0000000..665cd7b
--- /dev/null
+++ b/solutions/2/q212/solution.go
@@ -0,0 +1,89 @@
+package q212
+
+type TrieNode struct {
+	children [26]*TrieNode
+	word     *string
+}
+
+func trieIdx(char byte) int8 {
+	if char < 'a' || char > 'z' {
+		return -1
+	}
+	return int8(char - 'a')
+}
+
+func allocator(bulkSize int) func() *TrieNode {
+	var v []TrieNode
+	p := 0
+
+	return func() *TrieNode {
+		if v == nil || p == bulkSize {
+			v = make([]TrieNode, bulkSize)
+			p = 0
+		}
+		p++
+		return &v[p-1]
+	}
+}
+
+func buildTrie(words []string) *TrieNode {
+	alloc := allocator(128)
+	root := alloc()
+	for _, word := range words {
+		p := root
+		for i := range len(word) {
+			idx := trieIdx(word[i])
+			if idx < 0 {
+				continue
+			}
+			if p.children[idx] == nil {
+				p.children[idx] = alloc()
+			}
+			p = p.children[idx]
+		}
+		p.word = &word
+	}
+	return root
+}
+
+type Coord [2]int8
+
+func find(board [][]byte, coord Coord, trie *TrieNode, found *[]string) {
+	if coord[0] < 0 || coord[1] < 0 ||
+		int(coord[0]) >= len(board) || int(coord[1]) >= len(board[0]) {
+		return
+	}
+	c := board[coord[0]][coord[1]]
+	idx := trieIdx(c)
+	if idx == -1 || trie.children[idx] == nil {
+		return
+	}
+	// Mark visited
+	board[coord[0]][coord[1]] = '.'
+
+	trie = trie.children[idx]
+	if trie.word != nil {
+		*found = append(*found, *trie.word)
+		trie.word = nil // dedup
+	}
+
+	find(board, Coord{coord[0] + 1, coord[1]}, trie, found)
+	find(board, Coord{coord[0] - 1, coord[1]}, trie, found)
+	find(board, Coord{coord[0], coord[1] + 1}, trie, found)
+	find(board, Coord{coord[0], coord[1] - 1}, trie, found)
+	board[coord[0]][coord[1]] = c
+}
+
+func findWords(board [][]byte, words []string) []string {
+	trie := buildTrie(words)
+	found := []string{}
+
+	for row := range board {
+		for col := range board[0] {
+			find(board, Coord{int8(row), int8(col)}, trie, &found)
+		}
+	}
+	return found
+}
+
+var _ = findWords
diff --git a/solutions/2/q221/solution.go b/solutions/2/q221/solution.go
new file mode 100644
index 0000000..f08d92a
--- /dev/null
+++ b/solutions/2/q221/solution.go
@@ -0,0 +1,26 @@
+package q221
+
+func maximalSquare(matrix [][]byte) int {
+	w, h := len(matrix[0]), len(matrix)
+
+	maxSq := make([]int16, w+h)
+	maxV := make([]int16, w)
+	ret := 0
+	for y := range h {
+		var maxH int16
+		for x := range w {
+			if matrix[y][x] == '0' {
+				maxH, maxV[x], maxSq[x-y+h] = 0, 0, 0
+				continue
+			}
+
+			maxH++
+			maxV[x]++
+			maxSq[x-y+h] = min(maxH, maxV[x], maxSq[x-y+h]+1)
+			ret = max(ret, int(maxSq[x-y+h]))
+		}
+	}
+	return ret * ret
+}
+
+var _ = maximalSquare
diff --git a/solutions/2/q230/solution.go b/solutions/2/q230/solution.go
new file mode 100644
index 0000000..6f4e362
--- /dev/null
+++ b/solutions/2/q230/solution.go
@@ -0,0 +1,35 @@
+package q230
+
+type TreeNode struct {
+	Val   int
+	Left  *TreeNode
+	Right *TreeNode
+}
+
+func findKth(node *TreeNode, k, current int) (int, *int) {
+	if node == nil {
+		return current, nil
+	}
+
+	current, ret := findKth(node.Left, k, current)
+	if ret != nil {
+		return current, ret
+	}
+
+	current += 1
+	if current == k {
+		return current, &node.Val
+	}
+
+	return findKth(node.Right, k, current)
+}
+
+func kthSmallest(root *TreeNode, k int) int {
+	_, kth := findKth(root, k, 0)
+	if kth == nil {
+		return 0
+	}
+	return *kth
+}
+
+var _ = kthSmallest
diff --git a/solutions/3/q322/solution.go b/solutions/3/q322/solution.go
new file mode 100644
index 0000000..81e85af
--- /dev/null
+++ b/solutions/3/q322/solution.go
@@ -0,0 +1,31 @@
+package q322
+
+// Note: The cache size only needs to be min(max(coins), amount).
+
+func coinChange(coins []int, amount int) int {
+	if amount == 0 {
+		return 0
+	}
+
+	cache := make([]int, amount+1)
+	cache[0] = amount + 1
+	for p := 1; p < len(cache); p *= 2 {
+		copy(cache[p:], cache[:p]) // fill with MAX+1
+	}
+	cache[0] = 0
+
+	for i := 1; i < len(cache); i++ {
+		for _, c := range coins {
+			if (i - c) < 0 {
+				continue
+			}
+			cache[i] = min(cache[i], cache[i-c]+1)
+		}
+	}
+	if cache[amount] > amount {
+		return -1
+	}
+	return cache[amount]
+}
+
+var _ = coinChange
diff --git a/solutions/4/q433/solution.go b/solutions/4/q433/solution.go
new file mode 100644
index 0000000..cd8b84e
--- /dev/null
+++ b/solutions/4/q433/solution.go
@@ -0,0 +1,71 @@
+package q433
+
+import "slices"
+
+func canMutate(a, b string) bool {
+	diff := false
+	for i := range len(a) {
+		if a[i] != b[i] {
+			if diff {
+				return false
+			}
+			diff = true
+		}
+	}
+	return diff // exactly one mutation
+}
+
+func minMutation(startGene string, endGene string, bank []string) int {
+	reachables := make([][]int, len(bank))
+	initial := []int{}
+	target := -1
+	for i := range len(bank) - 1 {
+		for j := i + 1; j < len(bank); j++ {
+			if canMutate(bank[i], bank[j]) {
+				reachables[i] = append(reachables[i], j)
+				reachables[j] = append(reachables[j], i)
+			}
+		}
+	}
+	for i := range bank {
+		if bank[i] == endGene {
+			target = i
+		}
+		if canMutate(startGene, bank[i]) {
+			if i == target {
+				return 1
+			}
+			initial = append(initial, i)
+		}
+	}
+	if target == -1 {
+		return -1
+	}
+
+	type qElem struct {
+		path []int
+	}
+	queue := make([]qElem, 0, len(initial)*2)
+	for _, i := range initial {
+		queue = append(queue, qElem{path: []int{i}})
+	}
+
+	for ; len(queue) > 0; queue = queue[1:] {
+		cur := queue[0].path
+		last := cur[len(cur)-1]
+		next := reachables[last]
+		for _, n := range next {
+			if n == target {
+				return len(cur) + 1
+			}
+			if slices.Contains(cur, n) {
+				continue
+			}
+			queue = append(queue, qElem{path: append(cur, n)})
+		}
+	}
+
+	return -1
+}
+
+var _ = minMutation
diff --git a/solutions/4/q452/solution.go b/solutions/4/q452/solution.go
new file mode 100644
index 0000000..e93b6b2
--- /dev/null
+++ b/solutions/4/q452/solution.go
@@ -0,0 +1,23 @@
+package q452
+
+import "slices"
+
+func findMinArrowShots(points [][]int) int {
+	slices.SortFunc(points, func(a, b []int) int { return a[0] - b[0] })
+	arrows := 1
+	prev := points[0]
+
+	for i := 1; i < len(points); i++ {
+		cur := points[i]
+		if cur[0] > prev[1] {
+			arrows++
+			prev = cur
+			continue
+		}
+		prev[0] = max(prev[0], cur[0])
+		prev[1] = min(prev[1], cur[1])
+	}
+	return arrows
+}
+
+var _ = findMinArrowShots