LEETCODE: Validate Binary Search Tree

验证二叉搜索树的有效性

最新推荐文章于 2025-09-10 14:02:06 发布

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#LEETCODE #算法 #递归 #二叉树

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本文介绍了一种检查给定二叉树是否为有效二叉搜索树的方法。通过递归方式检查每个节点，确保左子树的所有节点值小于根节点值，而右子树的所有节点值大于根节点值。

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.

二叉树的性质，各种遍历方法。好像中序遍历是先遍历左子树，在根节点，最后右节点。根几点大于左子树所有节点，并且最大的在左子树中最靠右的。要记住这个位置！

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
private:
    TreeNode *lastOfLeft;
    bool checkValid(TreeNode *root) {
        if(root == NULL)    return true;
        bool isLeftOK = checkValid(root->left);
        if(lastOfLeft != NULL && root->val <= lastOfLeft->val) {
            return false;
        }
        lastOfLeft = root;
        bool isRightOK = checkValid(root->right);
        return isLeftOK && isRightOK;
    }
public:
    bool isValidBST(TreeNode *root) {
        if(root == NULL)    return true;
        lastOfLeft = NULL;
        return checkValid(root);
        
    }
};