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.

 /**
  * 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) {
          return isValidBST(root, LONG_MIN, LONG_MAX);
     }
     bool isValidBST(TreeNode *root, long mn, long mx) {
         if (!root) return true;
         if (root->val <= mn || root->val >= mx) return false;
         return isValidBST(root->left, mn, root->val) && isValidBST(root->right, root->val, mx);
     }
 };