[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?

Analysis:

Pretty straightforward. Push right and left tree into the stack. 

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