144. 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]

.

Note: Recursive solution is trivial, could you do it iteratively?
题意：迭代实现栈的先序遍历。
思路：使用栈。先读取，再入栈。

/**
* 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) {
vector<int> res;
stack<TreeNode*> mystack;
TreeNode *p = root;
while (p){
res.push_back(p->val);
mystack.push(p);
p = p->left;
}
while (!mystack.empty()){
TreeNode* q = mystack.top();
mystack.pop();
q = q->right;
while (q){
res.push_back(q->val);
mystack.push(q);
q = q->left;
}
}
return res;
}
};