力扣hot100 - 98、验证二叉搜索树

题目：思路：二叉搜索树如果按照中序遍历，那么遍历的结果一定是递增的。我们那一个pre指针指向遍历元素的前一个元素，不断比较更新当前元素和前一个元素的值。

/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode() {} * TreeNode(int val) { this.val = val; } * TreeNode(int val, TreeNode left, TreeNode right) { * this.val = val; * this.left = left; * this.right = right; * } * } */ class Solution { public TreeNode pre = null; public boolean isValidBST(TreeNode root) { if(root == null) return true; boolean l = isValidBST(root.left); if(pre != null && root.val <= pre.val){ return false; } pre = root; boolean r = isValidBST(root.right); return l && r; } }