/*
 * @lc app=leetcode.cn id=98 lang=javascript
 *
 * [98] 验证二叉搜索树
 */

// @lc code=start
/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
function TreeNode(val, left, right) {
  this.val = val === undefined ? 0 : val;
  this.left = left === undefined ? null : left;
  this.right = right === undefined ? null : right;
}

let root = new TreeNode(
  5,
  new TreeNode(4, undefined, undefined),
  new TreeNode(
    6,
    new TreeNode(3, undefined, undefined),
    new TreeNode(7, undefined, undefined)
  )
);

/**
 * @param {TreeNode} root
 * @return {boolean}
 */
let pre = null;
var isValidBST = function (root) {
  let pre = null;
  const inOrder = (root) => {
    if (root === null) {
      return true;
    }
    let left = inOrder(root.left);
    if (pre !== null && pre.val >= root.val) {
      return false;
    }
    pre = root;

    let right = inOrder(root.right);
    return left && right;
  };
  return inOrder(root);
};
isValidBST(root);
// @lc code=end