Binary Tree Level Order Traversal (LeetCode 102) – Problem Description and Solutions in Java, C++, C, and Python

The article begins with a humorous observation about competition between two Huawei cafeterias, likening it to market competition and noting the impact of performance rankings.

It then introduces LeetCode problem 102, which asks for a level‑order (breadth‑first) traversal of a binary tree, providing the problem description, input and output examples, and constraints on node values and tree size.

After the problem statement, detailed solution code is presented in four languages—Java, C++, C, and Python—each using a queue to perform BFS, count nodes per level, and collect values into a list of lists representing each tree level.

All code snippets are shown in full and wrapped in

tags to preserve their original formatting.</p><p>Java solution:</p><code>public List<List<Integer>> levelOrder(TreeNode root) {
    List<List<Integer>> ans = new LinkedList<>();
    if (root == null)
        return ans;
    Queue<TreeNode> queue = new LinkedList<>(); // 创建队列
    queue.offer(root); // 把根节点添加到队列中
    while (!queue.isEmpty()) {
        int levelCount = queue.size(); // 每层的节点个数
        List<Integer> subList = new ArrayList<>();
        for (int i = 0; i < levelCount; i++) {
            TreeNode cur = queue.poll(); // 节点出队
            subList.add(cur.val);
            if (cur.left != null)
                queue.offer(cur.left);
            if (cur.right != null)
                queue.offer(cur.right);
        }
        ans.add(subList); // 当前层的集合添加到队列中。
    }
    return ans;
}

C++ solution:

vector<vector<int>> levelOrder(TreeNode *root) {
    vector<vector<int>> ans;
    if (root == nullptr)
        return ans;
    queue<TreeNode *> q; // 创建队列
    q.push(root); // 把根节点添加到队列中
    while (!q.empty()) {
        int levelCount = q.size(); // 每层的节点个数
        vector<int> subList;
        for (int i = 0; i < levelCount; i++) {
            TreeNode *cur = q.front();
            q.pop(); // 节点出队
            subList.push_back(cur->val);
            if (cur->left)
                q.push(cur->left);
            if (cur->right)
                q.push(cur->right);
        }
        ans.push_back(subList); // 当前层的集合添加到队列中。
    }
    return ans;
}

C solution:

int **levelOrder(struct TreeNode *root, int *returnSize, int **returnColumnSizes) {
    *returnSize = 0;
    int **ans = (int **)malloc(sizeof(int *) * 2001);
    if (root == NULL)
        return ans;
    *returnColumnSizes = malloc(sizeof(int) * 2001);
    struct TreeNode **q = (struct TreeNode **)malloc(sizeof(struct TreeNode *) * 2001); // 创建队列
    int left = 0, right = 0; // 左闭右开
    q[right++] = root; // 把根节点添加到队列中
    while (left != right) {
        int levelCount = right - left; // 每层的节点个数
        int *subList = (int *)malloc(sizeof(int) * levelCount);
        for (int i = 0; i < levelCount; i++) {
            struct TreeNode *cur = q[left++]; // 节点出队
            subList[i] = cur->val;
            if (cur->left)
                q[right++] = cur->left;
            if (cur->right)
                q[right++] = cur->right;
        }
        (*returnColumnSizes)[*returnSize] = levelCount;
        ans[(*returnSize)++] = subList; // 当前层的集合添加到队列中。
    }
    return ans;
}

Python solution:

def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
    ans = []
    if root is None:
        return ans
    q = deque([root])  # 创建队列
    while q:
        levelCount = len(q)  # 每层的节点个数
        subList = []
        for _ in range(levelCount):
            cur = q.popleft()  # 节点出队
            subList.append(cur.val)
            if cur.left:
                q.append(cur.left)
            if cur.right:
                q.append(cur.right)
        ans.append(subList)  # 当前层的集合添加到队列中。
    return ans