[LeetCode] Binary Tree Preorder Traversal

Link : http://oj.leetcode.com/problems/binary-tree-preorder-traversal/

/**
* 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;
        std::stack<TreeNode*> ss;
        ss.push(root);
        TreeNode* node;
        while(ss.size()>0)
        {
            node = ss.top();
            ss.pop();
            ret.push_back(node->val);
            if(node->right)
                ss.push(node->right);
            if(node->left)
                ss.push(node->left);
            
        }
        return ret;
    }
};