leetcode-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.

/**
* 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)
{
if(NULL==root)
return true;
TreeNode *p,*q;
if(isValidBST(root->left)&&isValidBST(root->right))
{
p = root->left;
q = root->right;
if(NULL!=p)
while(p->right)p=p->right;
if(NULL!=q)
while(q->left)q=q->left;
if(p&&p->val>=root->val)return false;
if(q&&q->val<=root->val)return false;
return true;
}
return false;
}
};