[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},

return [1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?

思路：用栈。之间复杂度O(n),空间复杂度O(n)

 1 /**
 2  * Definition for binary tree
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     vector<int> preorderTraversal(TreeNode *root) {
13         vector<int> result;
14         stack<TreeNode*> s;
15         TreeNode *p = root;
16         
17         while (p != NULL || !s.empty()) {
18             if (p != NULL) {
19                 result.push_back(p->val);
20                 s.push(p);
21                 p = p->left;
22             } else {
23                 p = s.top();
24                 s.pop();
25                 p = p->right;
26             }
27         }
28          return result;
29     }
30 };