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

1 解题思想

这题我的做法，直接就是先序遍历，遍历过程中，检查是否出错了，代码很简单。

注意用于上一个值追踪的last，一开始必须要比Int的最小值小才行，所以用了long

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   3Binary tree [2,1,3], return true.Example 2:    1   / \  2   3Binary 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) ;    } }