Leecode_98_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.

  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 };