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?

/** * 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) {        // IMPORTANT: Please reset any member data you declared, as        // the same Solution instance will be reused for each test case.        vector<int> nodes;        stack<TreeNode*> s;        TreeNode *p = root;        if(p != NULL)        {            s.push(p);            while(!s.empty())            {                p = s.top();                s.pop();                nodes.push_back(p->val);                if(p->right != NULL)                    s.push(p->right);                if(p->left != NULL)                    s.push(p->left);            }        }                return nodes;    }};