update

medium/331.verify-preorder-serialization-of-a-binary-tree.go

@@ -0,0 +1,37 @@
+func isValidSerialization(preorder string) bool {
+	tree := strings.Split(preorder, ",")
+	n := len(tree)
+	if n == 0 || (n == 1 && tree[0] != "#") || (tree[0] == "#" && 1 < n) {
+		return false
+	}
+	st := []int{-1} // 0 for left, and 1 for right
+	l, i, next := 1, 1, 0
+	for ; i < n && l != 0; i++ {
+		if next == 0 {
+			st = append(st, 0)
+			l++
+			if tree[i] == "#" {
+				next = 1
+			}
+		} else {
+			if tree[i] == "#" {
+				l--
+				st = st[:l]
+				for 2 < l && st[l-1] == 1 {
+					l--
+					if st[l-1] != 0 {
+						return false
+					}
+					l--
+					st = st[:l]
+				}
+			} else {
+				st = append(st, 1)
+				l++
+				next = 0
+			}
+		}
+	}
+	return len(st) == 1 && i == n
+}
+