从大到小遍历一遍，每次取M个元素，然后求得最小的floor即可
int minimum(int M, vector <int> heights)
{
sort(heights.begin(),heights.end());
int minFloor = 10000;
for(int i = heights.size()-1; i >=M-1; -- i){
int floor = 0;
for(int j = 0; j < M; ++j) floor +=heights[i]-heights[i-j];
minFloor =min(minFloor,floor);
}
return minFloor;
}