144. Binary Tree Preorder Traversal

iven 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]

.

solution:

/**
* Definition for a binary tree node.
* 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) {
if(!root) return {};
vector<int> res;
stack<TreeNode*> stk;
stk.push(root);
while(!stk.empty()){
TreeNode *tmp = stk.top();
res.push_back(tmp->val);
stk.pop();
if(tmp->right) stk.push(tmp->right);
if(tmp->left)  stk.push(tmp->left);
}
return res;
}
};

思路：用栈的地方基本可以用递归， 主要考虑的是循环的终止时间

运行速度：快