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.
1 /** 2 * Definition for a binary tree node. 3 * struct TreeNode { 4 * int val; 5 * TreeNode *left; 6 * TreeNode *right; 7 * TreeNode(int x) : val(x), left(NULL), right(NULL) {} 8 * }; 9 */ 10 class Solution { 11 public: 12 bool isValidBST(TreeNode* root) { 13 return isValidBST(root, LONG_MIN, LONG_MAX); 14 } 15 bool isValidBST(TreeNode *root, long mn, long mx) { 16 if (!root) return true; 17 if (root->val <= mn || root->val >= mx) return false; 18 return isValidBST(root->left, mn, root->val) && isValidBST(root->right, root->val, mx); 19 } 20 };