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

递归即可。

/** * Definition for binary tree * 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 checkValidBST(root, numeric_limits<int>::min(), numeric_limits<int>::max());    }        bool checkValidBST(TreeNode *root, int min_val, int max_val) {        if (root == NULL) {            return true;        }                bool valid = false;        if (root->val >= min_val && root->val <= max_val) {            valid = checkValidBST(root->left, min_val, root->val - 1);            if (valid) {                valid = checkValidBST(root->right, root->val + 1, max_val);            }        }                return valid;    }};

============================第二次============================

/** * Definition for binary tree * 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 checkBST(root, numeric_limits<int>::min(), numeric_limits<int>::max());    }        bool checkBST(TreeNode *root, int left, int right) {        if (root == NULL) {            return true;        }                if (root->val <= right && root->val >= left) {            return checkBST(root->left, left, root->val - 1)                     && checkBST(root->right, root->val + 1, right);        }                return false;    }};