Largest Number Subsequences of K Digits from Sequence

google Given a sequence S of N digits, find a subsequence of K digits such that the number formed by these K digits (in order) is the largest. Examples given: S = "1", K = 1, answer = "1" # Single digit S = "10", K = 1, answer = "1" # Zero included S = "0000", K = 2, answer = "00" # All zeros S = "1000", K = 3, answer = "100" # Leading zeros More examples: S = "98765", K = 2, answer = "98" S = "98765", K = 3, answer = "987" S = "98765", K = 4, answer = "9876" S = "19191", K = 2, answer = "91" S = "19191", K = 3, answer = "991" S = "19191", K = 4, answer = "9191" Example Input S = "1", K = 1