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Leetcode #1424: Diagonal Traverse II
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.
return answer;
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