Kth Smallest Element in a Sorted Matrix

class Solution {
    int count(vector<vector<int>>& matrix,int val){
        int c=0,n=matrix.size(),lb;
        for(int i=0;i<n;++i){
            lb = upper_bound(matrix[i].begin(),matrix[i].end(),val)-matrix[i].begin();
            c += lb;
        }
        return c;
    }
public:
    int kthSmallest(vector<vector<int>>& matrix, int k) {
        int n=matrix.size();
        int low=matrix[0][0],high=matrix[n-1][n-1],mid;
         while(low < high){
            mid = low+(high-low)/2;
            int c = count(matrix,mid);
            if(c < k)   low=mid+1;
            else        high=mid;
        }
        return low;
    }
};

good solution