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.

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/**
 * 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(root == NULL)
			return true;
        vector<int> res;
		pre_search(root,res);
		for(int i=1; i<res.size(); i++)
		{
			if(res[i] <= res[i-1])
				return false;
		}
		return true;
    }
	void pre_search(TreeNode* root, vector<int>& res)
	{
		if(root == NULL)
			return;
		pre_search(root->left,res);
		res.push_back(root->val);
		pre_search(root->right,res);
	}	
};

LeetCode--Validate Binary Search Tree