1.题目解析

2.讲解算法原理 

二叉搜索树的中序遍历是一个有序序列

1.使用全局变量的优势

2.回溯

3.剪枝

• 左子树是二叉搜索树
• 当前节点符合二叉搜索树的定义
• 右子树是一个二叉搜索树

3.编写代码 

/*** 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 long prev=Long.MIN_VALUE;;public boolean isValidBST(TreeNode root) {if(root==null){return true;}boolean leftnode=isValidBST(root.left);boolean cur=false;if(root.val>prev){cur=true;}prev=root.val;boolean rightnode=isValidBST(root.right);return leftnode&&rightnode&&cur;}
}