Validate Binary Search Tree

題目：

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.
分析：先用中序遍歷記下所有節點，然後判斷此數組是否是已排序的。

代碼如下：

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