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【leetcode刷题笔记】Validate Binary Search Tree

2024-07-12 22:22:01   222人阅读

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

  • The left subtree of a node contains only nodes with keys less than the node‘s key.
  • The right subtree of a node contains only nodes with keys greater than the node‘s key.
  • Both the left and right subtrees must also be binary search trees.

 

题解：BST valid 的充分必要条件是它的中序遍历是一个有序序列。

递归实现树的中序遍历，用私有变量lastVal记录上一个遍历的节点的值。在一次递归，首先递归判断左子树是否是BST，并且更新lastVal，然后将root的值跟lastVal比较，看root的值是否大于lastVal；然后递归判断右子树是否是BST。

代码如下：

 1 /** 2  * Definition for binary tree 3  * public class TreeNode { 4  *     int val; 5  *     TreeNode left; 6  *     TreeNode right; 7  *     TreeNode(int x) { val = x; } 8  * } 9  */10 public class Solution {11     private int lastVal = Integer.MIN_VALUE;12     public boolean isValidBST(TreeNode root) {13         if(root == null)14             return true;15         16         if(!isValidBST(root.left))17             return false;18         19         if(root.val <= lastVal)20             return false;21         lastVal = root.val;22         if(!isValidBST(root.right))23             return false;24         return true;25     }26 }

题目的关键点是lastVal更新的时机和与root比较的时机。


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