add new solutions

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