Leetcode 98. Validate Binary Search Tree (Medium) (cpp)

Leetcode 98. Validate Binary Search Tree (Medium) (cpp)

Tag: Tree, Depth-first Search

Difficulty: Medium

/*98. Validate Binary Search Tree (Medium)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   3Binary tree [2,1,3], return true.Example 2:    1   / \  2   3Binary tree [1,2,3], return false.*//** * 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) {        TreeNode* prev = NULL;        return validate(root, prev);    }private:    bool validate(TreeNode* root, TreeNode* &prev) {        if (root == NULL) {            return true;        } else if (!validate(root->left, prev)) {            return false;        } else if (prev != NULL && prev->val >= root->val) {            return false;        }        prev = root;        return validate(root->right, prev);    }};