LeetCode——Binary Tree Preorder Traversal

题目：

Given a binary tree, return the preorder traversal of its nodes' values.For example:Given binary tree {1,#,2,3},   1    \     2    /   3return [1,2,3].

解答：

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