LeetCode *** 144. Binary Tree Preorder Traversal

题目：

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?

分析：

利用堆栈进行先序遍历即可。

代码：

/**
* 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;

if(root==NULL)return res;

stack<TreeNode*> tree;
tree.push(root);
while(!tree.empty()){
TreeNode* tmp=tree.top();
tree.pop();
res.push_back(tmp->val);
if(tmp->right)tree.push(tmp->right);
if(tmp->left)tree.push(tmp->left);
}
return res;
}
};