Add elements of 2d array in C++

You can do that by, for example,

1. Calculate the maximum number of elements of the elements of arr.
2. Iterate from 0 to the maximum number minus one.
3. Retrieve the ith element counted from the last element with checking if the element has i element or more.
4. Add the elements.

#include <iostream>
#include <vector>
using std::vector;

int main(void) {
    vector<vector<int>> arr = {{1, 1}, 
                               {1, 3, 2}, 
                               {1, 6, 11, 6}};

    size_t max_num = 0;
    for (size_t i = 0; i < arr.size(); i++) {
        if (max_num < arr[i].size()) max_num = arr[i].size();
    }
    for (size_t i = 0; i < max_num; i++) {
        int sum = 0;
        for (size_t j = 0; j < arr.size(); j++) {
            if (i < arr[j].size()) sum += arr[j][arr[j].size() - 1 - i];
        }
        std::cout << sum << '\n';
    }
    return 0;
}