LeetCode 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?

给定一棵二叉树，返回树的前序遍历结果。

例如：

给定二叉树 {1,#,2,3}

   1    \     2    /   3

返回 [1,2,3]

注意：仅用递归解决是不够的，你能用迭代解决吗？

写完之后才发现Note里面要用迭代。。。不过这一问把我问得有点迷糊了，感觉自己写的应该是算递归的，但是怎么样才算迭代呢？

/** * 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> ans;        if(!root) return ans;        traval(ans,root);        return ans;    }    void traval(vector<int>& ans,TreeNode* root){        ans.push_back(root->val);        if(root->left) traval(ans,root->left);        if(root->right) traval(ans,root->right);    }};