Leetcode #1424: Diagonal Traverse II

Leetcode_1424.cpp

vector<int> findDiagonalOrder(vector<vector<int>>& nums) {
    vector<int> answer;
    unordered_map<int, vector<int>> m;
    int maxKey = 0; // maximum key inserted into the map i.e. max value of i+j indices.

    for (int i=0; i<nums.size(); i++) {
        for (int j=0; j<nums[i].size(); j++) {
            m[i+j].push_back(nums[i][j]); // insert nums[i][j] in bucket (i+j).
            maxKey = max(maxKey, i+j); // 
        }
    }
    for (int i=0; i<= maxKey; i++) { // Each diagonal starting with sum 0 to sum maxKey.
        for (auto x = m[i].rbegin(); x != m[i].rend(); x++) { // print in reverse order.
            answer.push_back(*x); 
        }
    }

    return answer;
}