diff --git a/solutions/0/q52/solution.go b/solutions/0/q52/solution.go
new file mode 100644
index 0000000..8618f23
--- /dev/null
+++ b/solutions/0/q52/solution.go
@@ -0,0 +1,24 @@
+package q52
+
+func findNSolutions(nQueen, row int, rows, cols, diag1, diag2 []bool) int {
+	solutions := 0
+	if row == nQueen {
+		return 1
+	}
+	for col := range nQueen {
+		// if grid[row][col] is placeable
+		if !rows[row] && !cols[col] && !diag1[row+col] && !diag2[row-col+nQueen] {
+			rows[row], cols[col], diag1[row+col], diag2[row-col+nQueen] = true, true, true, true
+			solutions += findNSolutions(nQueen, row+1, rows, cols, diag1, diag2)
+			rows[row], cols[col], diag1[row+col], diag2[row-col+nQueen] = false, false, false, false
+		}
+	}
+	return solutions
+}
+
+func totalNQueens(n int) int {
+	buf := make([]bool, 6*n)
+	return findNSolutions(n, 0, buf[0:n], buf[n:2*n], buf[2*n:4*n], buf[4*n:])
+}
+
+var _ = totalNQueens
diff --git a/solutions/0/q63/solution.go b/solutions/0/q63/solution.go
new file mode 100644
index 0000000..cf5011e
--- /dev/null
+++ b/solutions/0/q63/solution.go
@@ -0,0 +1,28 @@
+package q63
+
+func uniquePathsWithObstacles(obstacleGrid [][]int) int {
+	w, h := len(obstacleGrid[0]), len(obstacleGrid)
+	if obstacleGrid[h-1][w-1]+obstacleGrid[0][0] > 0 {
+		return 0
+	}
+
+	obstacleGrid[h-1][w-1] = -1
+	for i := w + h - 2; i >= 0; i-- {
+		for x := min(i, w-1); x >= 0 && i-x < h; x-- {
+			y := i - x
+			if obstacleGrid[y][x] == 1 {
+				continue
+			}
+			if x+1 < w && obstacleGrid[y][x+1] != 1 {
+				obstacleGrid[y][x] += obstacleGrid[y][x+1]
+			}
+			if y+1 < h && obstacleGrid[y+1][x] != 1 {
+				obstacleGrid[y][x] += obstacleGrid[y+1][x]
+			}
+		}
+	}
+
+	return -1 * obstacleGrid[0][0]
+}
+
+var _ = uniquePathsWithObstacles
diff --git a/solutions/1/q102/solution.go b/solutions/1/q102/solution.go
new file mode 100644
index 0000000..ab5828e
--- /dev/null
+++ b/solutions/1/q102/solution.go
@@ -0,0 +1,43 @@
+package q102
+
+type TreeNode struct {
+	Val   int
+	Left  *TreeNode
+	Right *TreeNode
+}
+
+func levelOrder(root *TreeNode) [][]int {
+	if root == nil {
+		return [][]int{}
+	}
+	ret := [][]int{}
+
+	type qElem struct {
+		lvl  int
+		node *TreeNode
+	}
+	curLvl := 0
+	curLvlNodes := []int{}
+	queue := []qElem{{0, root}}
+	for ; len(queue) > 0; queue = queue[1:] {
+		cur := &queue[0]
+		if cur.lvl != curLvl {
+			ret = append(ret, curLvlNodes)
+			curLvlNodes = []int{cur.node.Val}
+			curLvl = cur.lvl
+		} else {
+			curLvlNodes = append(curLvlNodes, cur.node.Val)
+		}
+		if cur.node.Left != nil {
+			queue = append(queue, qElem{lvl: cur.lvl + 1, node: cur.node.Left})
+		}
+		if cur.node.Right != nil {
+			queue = append(queue, qElem{lvl: cur.lvl + 1, node: cur.node.Right})
+		}
+	}
+
+	ret = append(ret, curLvlNodes)
+	return ret
+}
+
+var _ = levelOrder
diff --git a/solutions/13/q1390/solution.go b/solutions/13/q1390/solution.go
new file mode 100644
index 0000000..dc3778a
--- /dev/null
+++ b/solutions/13/q1390/solution.go
@@ -0,0 +1,45 @@
+package q1390
+
+import "slices"
+
+func sfd(n int) int {
+	if n < 6 {
+		return 0
+	}
+
+	buf := [6]int{1, n}
+	divisors := buf[:2]
+	nn := n
+	for i := 2; i*i <= nn; i++ {
+		for nn%i == 0 {
+			nn /= i
+
+			if !slices.Contains(divisors, i) {
+				divisors = append(divisors, i)
+			}
+			if !slices.Contains(divisors, nn) {
+				divisors = append(divisors, nn)
+			}
+			if !slices.Contains(divisors, n/nn) {
+				divisors = append(divisors, n/nn)
+			}
+			if len(divisors) > 4 {
+				return 0
+			}
+		}
+	}
+	if len(divisors) == 4 {
+		return divisors[0] + divisors[1] + divisors[2] + divisors[3]
+	}
+	return 0
+}
+
+func sumFourDivisors(nums []int) int {
+	sum := 0
+	for _, n := range nums {
+		sum += sfd(n)
+	}
+	return sum
+}
+
+var _ = sumFourDivisors
diff --git a/solutions/14/q1411/solution.go b/solutions/14/q1411/solution.go
new file mode 100644
index 0000000..d10f409
--- /dev/null
+++ b/solutions/14/q1411/solution.go
@@ -0,0 +1,62 @@
+package q1411
+
+type Color uint8
+
+const MODULO int = 1e9 + 7
+
+func numOfWays(n int) int {
+	patterns := make([][3]Color, 0, 12)
+	for a := range 3 {
+		for b := range 3 {
+			for c := range 3 {
+				if a == b || b == c {
+					continue
+				}
+				patterns = append(patterns, [3]Color{Color(a), Color(b), Color(c)})
+			}
+		}
+	}
+
+	// Find compatible patterns
+	compatiblePtns := make([][]int, len(patterns))
+	for i := range len(patterns) - 1 {
+		for j := i + 1; j < len(patterns); j++ {
+			ok := true
+			for k := range 3 {
+				if patterns[i][k] == patterns[j][k] {
+					ok = false
+					break
+				}
+			}
+			if ok {
+				compatiblePtns[i] = append(compatiblePtns[i], j)
+				compatiblePtns[j] = append(compatiblePtns[j], i)
+			}
+		}
+	}
+
+	counts := make([]int, len(patterns))
+	for i := range counts {
+		counts[i] = 1
+	}
+
+	countsNext := make([]int, len(patterns))
+	for range n - 1 {
+		for i := range len(patterns) {
+			countsNext[i] = 0
+			for _, j := range compatiblePtns[i] {
+				countsNext[i] += counts[j]
+			}
+			countsNext[i] %= MODULO
+		}
+		counts, countsNext = countsNext, counts
+	}
+
+	sum := 0
+	for _, c := range counts {
+		sum += c
+	}
+	return sum % MODULO
+}
+
+var _ = numOfWays
diff --git a/solutions/19/q1970/solution.go b/solutions/19/q1970/solution.go
new file mode 100644
index 0000000..76f2b88
--- /dev/null
+++ b/solutions/19/q1970/solution.go
@@ -0,0 +1,67 @@
+package q1970
+
+var dirs = [4][2]int{{0, 1}, {0, -1}, {1, 0}, {-1, 0}}
+
+func isPossibleOnDay(row, col int, cells [][]bool, day int, floods [][]int) bool {
+	for i := range col {
+		cells[0][i] = true
+	}
+	for i := range row - 1 {
+		copy(cells[i+1], cells[0])
+	}
+	for i := range day {
+		cells[floods[i][0]-1][floods[i][1]-1] = false
+	}
+
+	// Use bfs to find a path from top to bottom
+	queue := make([][2]int, 0, col) // {row, col}
+	for x, ok := range cells[0] {
+		if ok {
+			queue = append(queue, [2]int{0, x})
+			cells[0][x] = false
+		}
+	}
+	for ; len(queue) > 0; queue = queue[1:] {
+		r, c := queue[0][0], queue[0][1]
+		for _, d := range dirs {
+			tr, tc := r+d[0], c+d[1]
+			if tr < 0 || tc < 0 || tr >= row || tc >= col {
+				continue
+			}
+			if !cells[tr][tc] {
+				continue
+			}
+			if tr == row-1 {
+				return true // reached bottom
+			}
+			cells[tr][tc] = false
+			queue = append(queue, [2]int{tr, tc})
+		}
+	}
+	return false
+}
+
+func latestDayToCross(row int, col int, cells [][]int) int {
+	walkableBuf := make([]bool, col*row)
+	walkable := make([][]bool, row)
+	for i := range row {
+		walkable[i] = walkableBuf[i*col : (i+1)*col]
+	}
+
+	// Use binary search to find the last day
+	l, r := 1, len(cells)+1
+	for l < r {
+		if l+1 == r {
+			return l
+		}
+		m := (l + r) / 2
+		if isPossibleOnDay(row, col, walkable, m, cells) {
+			l = m
+		} else {
+			r = m
+		}
+	}
+	return 0 // impossible
+}
+
+var _ = latestDayToCross
diff --git a/solutions/2/q226/solution.go b/solutions/2/q226/solution.go
index 6e51053..0abc04f 100644
--- a/solutions/2/q226/solution.go
+++ b/solutions/2/q226/solution.go
@@ -15,3 +15,5 @@ func invertTree(root *TreeNode) *TreeNode {
 	invertTree(root.Right)
 	return root
 }
+
+var _ = invertTree
diff --git a/solutions/3/q380/solution.go b/solutions/3/q380/solution.go
index 98a7857..685279e 100644
--- a/solutions/3/q380/solution.go
+++ b/solutions/3/q380/solution.go
@@ -1,13 +1,6 @@
 package q380
 
-import (
-	"math/rand"
-	"runtime/debug"
-)
-
-func init() {
-	debug.SetMemoryLimit(16 << 20)
-}
+import "math/rand"
 
 var rng *rand.Rand = rand.New(rand.NewSource(42))
 
diff --git a/solutions/8/q840/solution.go b/solutions/8/q840/solution.go
new file mode 100644
index 0000000..1a4d2f7
--- /dev/null
+++ b/solutions/8/q840/solution.go
@@ -0,0 +1,63 @@
+package q840
+
+func checkSquare(x, y int, grid [][]int) bool {
+	seen := uint16(0)
+	rowSum, colSum := [3]int{}, [3]int{}
+	diag1sum, diag2sum := 0, 0
+
+	for ty := range 3 {
+		for tx := range 3 {
+			num := grid[y+ty][x+tx]
+			if num < 1 || num > 9 {
+				return false
+			}
+			tSeen := seen | (1 << (num - 1))
+			if tSeen == seen { // duplicate number
+				return false
+			}
+			seen = tSeen
+
+			rowSum[ty] += num
+			colSum[tx] += num
+			if tx == ty {
+				diag1sum += num
+			}
+			if tx+ty == 2 {
+				diag2sum += num
+			}
+		}
+	}
+
+	if diag1sum != diag2sum {
+		return false
+	}
+	for i := range 3 {
+		if rowSum[i] != diag1sum || colSum[i] != diag1sum {
+			return false
+		}
+	}
+	return true
+}
+
+func numMagicSquaresInside(grid [][]int) int {
+	if len(grid) < 3 || len(grid[0]) < 3 {
+		return 0
+	}
+	w, h := len(grid[0]), len(grid)
+
+	count := 0
+	for y := range h - 2 {
+		for x := range w - 2 {
+			if grid[y+1][x+1] != 5 { // the center cell has to be 5
+				continue
+			}
+			if checkSquare(x, y, grid) {
+				count++
+			}
+		}
+	}
+
+	return count
+}
+
+var _ = numMagicSquaresInside
diff --git a/solutions/9/q961/solution.go b/solutions/9/q961/solution.go
new file mode 100644
index 0000000..f689a78
--- /dev/null
+++ b/solutions/9/q961/solution.go
@@ -0,0 +1,14 @@
+package q961
+
+import "math/rand"
+
+func repeatedNTimes(nums []int) int {
+	for {
+		a, b := rand.Intn(len(nums)), rand.Intn(len(nums))
+		if b != a && nums[a] == nums[b] {
+			return nums[a]
+		}
+	}
+}
+
+var _ = repeatedNTimes