Validate Binary Search Tree

题目：

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：先用中序遍历记下所有节点，然后判断此数组是否是已排序的。

代码如下：

void getroot(TreeNode *root,vector &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 result;
getroot(root,result);
for(int i=1;i {
if(result[i-1]>=result[i])
{
return false;
}
}
return true;
}