Leetcode 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?

递归大法代码如下：

/** * 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> result;        preorder(root,result);        return result;    }        void preorder(TreeNode* root,vector<int>& result)    {        if(root == NULL)            return;        result.push_back(root->val);        preorder(root->left,result);        preorder(root->right,result);    }};

迭代大法代码如下：

/** * 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> result;        stack<TreeNode*> data;        while(root)        {            if(root->right)                data.push(root->right);            result.push_back(root->val);            root = root->left;            if(root == NULL && !data.empty())            {                root = data.top();                data.pop();            }        }        return result;    }};