A C++ solution to https://leetcode.com/problems/the-k-weakest-rows-in-a-matrix/

solution.cpp

class Solution {
public:
    int soldiers(const vector<int>& row) {
        // Need to use greater<int>() since the vector is descending, not assending
        // It also explains lower_bound of 0 instead of 1 or 0.5
        auto it = lower_bound(row.begin(), row.end(), 0, greater<int>());
        return it-row.begin();
    }
        
    vector<int> kWeakestRows(vector<vector<int>>& mat, int k) {
        vector<pair<int, int> > weakness;
        weakness.reserve(mat.size());
        
        for (int i=0; i<mat.size(); i++)
            weakness.push_back(pair<int, int>(soldiers(mat[i]), i));
        
        std::partial_sort(weakness.begin(), weakness.begin() + k, weakness.end());

        vector<int> weakest;
        weakest.reserve(k);
        
        for (int i=0; i<k; i++)
            weakest.push_back(weakness[i].second);

        return weakest;
    }
};