【LeetCode】 98. Validate Binary Search Tree 验证二叉搜索树(Medium)(JAVA)
题目地址: https://leetcode.com/problems/validate-binary-search-tree/
题目描述:
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
Input: [2,1,3]
Output: true
Example 2:
5
/ \
1 4
/ \
3 6
Input: [5,1,4,null,null,3,6]
Output: false
Explanation: The root node's value is 5 but its right child's value is 4.
题目大意
给定一个二叉树,判断其是否是一个有效的二叉搜索树。
假设一个二叉搜索树具有如下特征:
- 节点的左子树只包含小于当前节点的数。
- 节点的右子树只包含大于当前节点的数。
- 所有左子树和右子树自身必须也是二叉搜索树。
解题方法
1、二叉搜素树的中序遍历是一个递增数组
2、对二叉树进行中序遍历,然后判断是否递增即可
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
Integer pre = null;
public boolean isValidBST(TreeNode root) {
if (root == null) return true;
boolean flag = isValidBST(root.left);
if (!flag) return false;
if (pre != null && pre >= root.val) return false;
pre = root.val;
flag = isValidBST(root.right);
if (!flag) return false;
return true;
}
}
执行用时 : 0 ms, 在所有 Java 提交中击败了100.00% 的用户
内存消耗 : 39.4 MB, 在所有 Java 提交中击败了 15.94% 的用户