Validate Binary Search Tree

/**
 * 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:
    void _inOrder(TreeNode* root, vector<int> &res) {

        if (NULL == root) {
            return;
        }
        
        _inOrder(root->left, res);
        res.push_back(root->val);
        _inOrder(root->right, res);
    }


    bool isValidBST(TreeNode* root) {
        vector<int> res;
        
        _inOrder(root, res);
        
        int i;
        for (i = 1; i< res.size(); i++) {
            if (res[i] <= res[i - 1]) {
                return false;
            }
        }
        return true;
    }
};