[Leetcode]Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes' values.

简单题，递归和迭代都可以解。

 1 class Solution {
 2 public:
 3     vector<int> preorderTraversal(TreeNode *root) {
 4         vector<int> val;
 5         if(root == NULL)
 6             return val;
 7         stack<TreeNode *> path;
 8         path.push(root);
 9         while(!path.empty()){
10             TreeNode* node = path.top();
11             path.pop();
12             val.push_back(node->val);
13 
14             if(node->right != NULL)
15             path.push(node->right);
16             if(node->left != NULL)
17             path.push(node->left);
18         }
19         return val;
20     }
21 };