Looking at it your method seems to be taking the lowest remaining value in the array each time (call this x), then subtracting x from the k-1 highest remaining values in the array to create x groups, repeating this process until no more groups can be created. In this case the greedy method fails in the following case:

5 3
2 2 2 2 2

In this case 3 groups can be made from boxes [1,2,3] , [2,3,4] , and [1,4,5]. In your current solution what is happening currently is that you take the lowest value 2, and end up creating two boxes of [1,2,5], leaving only two types of candies left. Technically this can be fixed by only making one box at a time and then resorting the array after each box, but the problem constraints (1 <= A[I] <= 10^9) would cause this to TLE. In practice the actual solution doesn't require you to explicitly determine how each group is created in the solution.