LeetCode 144. Binary Tree Preorder Traversal

 1 /**
 2  * Definition for a binary tree node.
 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> res;
13     void help(TreeNode* root){
14         // if(!root) return;
15         res.push_back(root->val);
16         if(root->left) help(root->left);
17         if(root->right) help(root->right);
18     }
19     vector<int> preorderTraversal(TreeNode* root) {
20         if(!root) return res;
21         help(root);
22         return res;
23     }
24     
25     
26 };

 1 /**
 2  * Definition for a binary tree node.
 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> res;
14         if(!root) return res;
15         stack<TreeNode*> stk;
16         stk.push(root);
17         
18         while(!stk.empty()){
19             TreeNode* cur = stk.top();
20             stk.pop();
21             res.push_back(cur->val);
22             
23             if(cur->right) stk.push(cur->right);
24             if(cur->left) stk.push(cur->left);
25             
26         }
27         return res;
28     }
29     
30 };