leetcode_Binary Tree Preorder Traversal

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

For example:

Given binary tree

{1,#,2,3}

,

1
\
2
/
3

return

[1,2,3]

.

/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
vector<int> preorderTraversal(TreeNode *root) {
vector <int> result;
stack <TreeNode * > s;
TreeNode *r = root;
while(r || !s.empty()){
if(r){
s.push(r);
result.push_back(r->val);
r=r->left;
}
else{
r = s.top();
s.pop();
r = r->right;
}

}
return  result;

}
};