50. Binary Tree Level Order Traversal

class Solution:
    def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
        if not root:
            return []
        res, q = [], deque([root])
        while q:
            level = []
            for _ in range(len(q)):        # exactly the nodes of this level
                node = q.popleft()
                level.append(node.val)
                if node.left: q.append(node.left)
                if node.right: q.append(node.right)
            res.append(level)
        return res

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if (root == null) return res;
        Queue<TreeNode> q = new ArrayDeque<>();
        q.add(root);
        while (!q.isEmpty()) {
            List<Integer> level = new ArrayList<>();
            for (int k = q.size(); k > 0; k--) {
                TreeNode node = q.poll();
                level.add(node.val);
                if (node.left != null) q.add(node.left);
                if (node.right != null) q.add(node.right);
            }
            res.add(level);
        }
        return res;
    }
}

class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> res;
        if (!root) return res;
        queue<TreeNode*> q;
        q.push(root);
        while (!q.empty()) {
            vector<int> level;
            for (int k = q.size(); k > 0; k--) {
                TreeNode* node = q.front(); q.pop();
                level.push_back(node->val);
                if (node->left) q.push(node->left);
                if (node->right) q.push(node->right);
            }
            res.push_back(level);
        }
        return res;
    }
};

function levelOrder(root) {
  if (!root) return [];
  const res = [];
  let level = [root];
  while (level.length) {
    res.push(level.map(n => n.val));
    const next = [];
    for (const n of level) { if (n.left) next.push(n.left); if (n.right) next.push(n.right); }
    level = next;
  }
  return res;
}