98. Validate Binary Search Tree(通过确定上下界进行递归)

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本文介绍了一种使用递归方式检查二叉树是否为有效二叉搜索树的方法，并通过定义节点的有效范围确保左子树小于根节点而右子树大于根节点。

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题目：

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.

代码：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isValidBST(TreeNode* root) {
        return isValidBST(root,nullptr,nullptr);
    }
    
    bool isValidBST(TreeNode *root,TreeNode *minode,TreeNode *maxnode){
        if(root==nullptr)
            return true;
        if((minode&&minode->val>=root->val)||(maxnode&&maxnode->val<=root->val))
            return false;
        return isValidBST(root->left,minode,root)&&isValidBST(root->right,root,maxnode);
    }
};