LeetCode(144)Binary Tree Preorder Traversal

题目如下:

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?分析如下:

和上一道题目类似，也是二叉树遍历的常规题目。鼓励用非递归的方式完成，而非递归的方式。因为递归相对比较简单。也因为以前写过。所以直接贴出leetcode上提交的代码

//8ms过大集合
/**
 * 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> res;
        stack<TreeNode*> stack_pre;
        if(root==NULL)
            return res;
        stack_pre.push(root);
        while(!stack_pre.empty()){
            TreeNode* top_node=stack_pre.top();
            res.push_back(top_node->val);
            stack_pre.pop();
            if(top_node->right!=NULL)
                stack_pre.push(top_node->right);
            if(top_node->left!=NULL)
                stack_pre.push(top_node->left);
        }
        return res;
       }
};