leetcode

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

For example:
Given binary tree {1,#,2,3},

return [1,2,3].

思路：前序遍历，“根，左子树，右子树”

 1 #include <iostream>
 2 #include <vector>
 3 #include <stack>
 4 using namespace std;
 5 
 6 struct TreeNode {
 7     int val;
 8     TreeNode *left;
 9     TreeNode *right;
10     TreeNode(int x): val(x), left(NULL), right(NULL) {}
11 };
12 
13 class Solution {
14 public:
15     vector<int> preorderTraversal(TreeNode *root) {
16         vector<int> res;
17         stack<TreeNode*> s;
18         if (!root) {
19             return res;
20         }
21 
22         s.push(root);
23         while(!s.empty()) {
24             TreeNode *p = s.top(); s.pop();
25             res.push_back(p->val);
26             if (p->right) {
27                 s.push(p->right);
28             }
29             if (p->left) {
30                 s.push(p->left);
31             }
32         }
33         return res;    
34     }
35 };
36 
37 int main() {
38     return 0;    
39 }