leetcode: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) {        stack<TreeNode *> searchStack;        vector<int> result;        if(root == NULL)            return result;        searchStack.push(root);        while(false == searchStack.empty())        {            TreeNode * cNode = searchStack.top();            searchStack.pop();            result.push_back(cNode->val);            if(cNode->right != NULL)            {                searchStack.push(cNode->right);            }            if(cNode->left != NULL)            {                searchStack.push(cNode->left);            }        }        return result;    }};