BFS，没啥好说的。

class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> ret;
        queue<pair<TreeNode*, int>> que;    // TreeNode*, depth
        if(root) que.push({root, 0});
        while(!que.empty()) {
            auto top = que.front(); que.pop();
            if((int)ret.size() - 1 < top.second) ret.push_back({top.first -> val});
            else {
                ret.back().emplace_back(top.first -> val);
            }
            if(top.first -> left) que.push({top.first -> left, top.second + 1});
            if(top.first -> right) que.push({top.first -> right, top.second + 1});
        }
        return ret;
    }
};