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;
    }
}