Leetcode 98. Validate Binary Search Tree 验证二叉搜索树 解题报告

1 解题思想

2 原题

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.
Example 1:
2
/ \
1   3
Binary tree [2,1,3], return true.
Example 2:
1
/ \
2   3
Binary tree [1,2,3], return false.

3 AC解

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
long last = Long.MIN_VALUE;
//采用左中右遍历后检查是否递增
public boolean isValidBST(TreeNode root) {
if (root == null) return true;
if ( !isValidBST(root.left) ) return false;
if (root.val <= last) return false;
last = root.val;
return isValidBST(root.right) ;
}
}