Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes' values.
Example
Given:

1
/ \
2   3
/ \
4   5

return 

[1,2,4,5,3]

.class Solution {
public:
vector<int> preorderTraversal(TreeNode* root) {
vector<int> res;
if(root==NULL)
return res;
stack<TreeNode*> s;
s.push(root);
while(!s.empty()){
TreeNode *t=s.top();
s.pop();
v.push_back(t->val);
if(t->right) //因为栈是先进后出，所以先放入右结点
s.push(t->right);
if(t->left)
s.push(t->left);
}
return res;
}
};