# 144. Binary Tree Preorder Traversal

## 1.問題

* 給予一個binary tree, 回傳pre-order traversal得到的值

![](https://901207480-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LGKoChvN9am4__HCIRK%2F-LIKpwpGmYYAbV2ledx7%2F-LIKqO48nWWSGwwXyG3z%2F2018072611.jpg?alt=media\&token=c3dc7f1a-1f8d-4223-b81a-0bed839f7838)

## 2.想法

* pre-order traversal

## 3.程式碼

```
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> v;
        preOrder(v, root);
        return v;
    }
private:
    void preOrder(vector<int>& v, TreeNode* root) {
        if (!root) {
            return;
        }
        v.push_back(root->val);
        preOrder(v, root->left);
        preOrder(v, root->right);
    }
};
```

## 4.Performance

![](https://901207480-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LGKoChvN9am4__HCIRK%2F-LIKpwpGmYYAbV2ledx7%2F-LIKqmXphpONDPmDlDVe%2F2018072612.jpg?alt=media\&token=a3343d37-6b57-499c-992b-e919ebb7915f)