LeetCode Validate Binary Search Tree_leetcode validatebinarysearchtree-优快云博客

本文介绍了一种简单而有效的递归方法来验证给定的二叉树是否为有效的二叉搜索树。通过定义一个辅助函数并设定每个节点的取值范围，确保了左子树的值小于根节点而右子树的值大于根节点。

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LeetCode Validate Binary Search Tree

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.

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思路分析：本题很简单，主要是要验证给定的二叉树是否是二叉搜索树，递归的solution比较好想到，主要是定义一个函数，其参数是某个treenode以及max和min,判断当前值是否在范围内。然后递归调用，左孩子的val应该在min和root.val之间，右孩子的val应该在root.val和max之间。

AC Code

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isValidBST(TreeNode root) {
        return validate(root, Integer.MAX_VALUE, Integer.MIN_VALUE);
    }
    
    public boolean validate(TreeNode root, int max, int min){
        if(root == null) {
            return true;
        }
        
        if(root.val >= max || root.val <= min){
            return false;
        }
        
        return validate(root.left, root.val, min) && validate(root.right, max, root.val);
    } 
}