[LeetCode] 129: Validate Binary Search Tree

[Problem]

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.
  

OJ's Binary Tree Serialization:

The serialization of a binary tree follows a level order traversal, where '#' signifies a path terminator where no node exists below.

Here's an example:

   1  / \ 2   3    /   4    \     5

The above binary tree is serialized as "{1,2,3,#,#,4,#,#,5}".

[Solution]

/**
 * 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, int leftVal, int rightVal){
// null node
if(NULL == root)return true;
return root->val > leftVal && root->val < rightVal && isValidBST(root->left, leftVal, root->val) && isValidBST(root->right, root->val, rightVal);
}
// check if a tree is a BST
bool isValidBST(TreeNode *root) {
// Note: The Solution object is instantiated only once and is reused by each test case.
return isValidBST(root, INT_MIN, INT_MAX);
}
};

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