Validate binary search tree(C++ code)

LeetCode Validate Binary Search Tree, Aug 31 '12
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.

思路：这题简单，一次bug free。。。最近都在科普python,sql ，为了避免C++手生，决定每天练一道leetcode，恩，发现C还是很重要的，把C搞懂了，学其他的才会事半功倍~

0. #helper function  
1. bool isValidBST(TreeNode *root, int lower, int upper){  
2.   
3.  if (!root) return true;  
4.   
5.  if(root->val <= lower || root->val >= upper) return false;  
6.   
7.  bool left = isValidBST(root->left, lower, root->val);  
8.   
9.  bool right = isValidBST(root->right, root->val, upper);  
10.   
11.    
12.   
13.  return left && right;  
14.   
15. }  
16.   
17. bool isValidBST(TreeNode *root) {  
18.         isValidBST(root, INT_MIN, INT_MAX);  
19.          
20.     }