add: #251 Validate Binary Search Tree

Author: sukyoungshin

validate-binary-search-tree/sukyoungshin.ts

@@ -0,0 +1,56 @@
+class TreeNode {
+  val: number;
+  left: TreeNode | null;
+  right: TreeNode | null;
+  constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
+    this.val = val === undefined ? 0 : val;
+    this.left = left === undefined ? null : left;
+    this.right = right === undefined ? null : right;
+  }
+}
+
+// 1번풀이
+function isValidBST1(root: TreeNode | null): boolean {
+  const values: number[] = [];
+
+  // 중위노드 : 왼쪽 자식 -> 부모 노드 -> 오른쪽 자식
+  function inorder(node: TreeNode | null) {
+    if (!node) return null;
+    const { val, left, right } = node;
+
+    inorder(left);
+    values.push(val);
+    inorder(right);
+  }
+
+  inorder(root);
+
+  for (let i = 1; i < values.length; i++) {
+    if (values[i] <= values[i - 1]) return false;
+  }
+
+  return true;
+};
+
+// 2번풀이 (배열 대신 변수 사용)
+function isValidBST2(root: TreeNode | null): boolean {
+  let prev: number | null = null;
+  let isValid = true;
+
+  function inorder(node: TreeNode | null) {
+    if (!node || !isValid) return;
+
+    inorder(node.left);
+
+    if (prev !== null && node.val <= prev) {
+      isValid = false;
+      return;
+    }
+    prev = node.val;
+
+    inorder(node.right);
+  }
+
+  inorder(root);
+  return isValid;
+};