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.

class Solution {
public:
    vector<int> rec;
    bool isValidBST(TreeNode* root) {
        if(root==NULL)
          return true;
        if(root->left==NULL&&root->right==NULL)
          return true;
        inOrder(root);
        for(int i=0;i<rec.size()-1;i++)
        {
            if(rec[i]>=rec[i+1])
              return false;
        }
        return true;
    }
    void inOrder(TreeNode* root)
    {
        if(root!=NULL)
        {
            inOrder(root->left);
            rec.push_back(root->val);
            inOrder(root->right);
        }
    }
};