> For the complete documentation index, see [llms.txt](https://jenhsuan.gitbook.io/algorithm/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://jenhsuan.gitbook.io/algorithm/leetcode/102.-binary-tree-level-order-traversal-medium.md).

# 102. Binary Tree Level Order Traversal

## 1.問題

* 給予一個binary tree, 回傳一個level order traversal的node's序列 (例如, 從左到右, 一層接一層)

![](/files/-LH2YSHtHJI40yN6YftE)

## 2.想法

* 可用level-order traversal, 廣度優先搜尋(BFS)的概念:
  * 1.先將root node放入一開始的queue
  * 2.queue當前中的每一個元素若有子樹的, 則加入queue中 (等待下一次)

## 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<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> v;
        if (!root) {
            return v;
        }
        queue<TreeNode*> q;
        vector<int> vTemp;
        vTemp.push_back(root->val);
        v.push_back(vTemp);
        q.push(root);
        
        while (!q.empty()){
            vector<int> vTemp;
            int size = q.size();
            for(int i = 0 ; i < size; i++){
                TreeNode* node = q.front();
                q.pop();
                if (node->left){
                    vTemp.push_back(node->left->val);
                    q.push(node->left);
                }
                if (node->right){
                    vTemp.push_back(node->right->val);
                    q.push(node->right);
                }
            }
            if (!vTemp.empty()){
                v.push_back(vTemp);
            }
        }
        return v;
    }
};
```

## 4.Performance

![](/files/-LH2YA-boZ-aNH8ywx54)


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