第十六周 6.13 --- 6.19

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