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);
}
};
笔记：
DFS类型的题目，通过上下界范围的确定进行递归的技巧
