How to find smallest substring which contains all characters from a given string?

Question

I have recently come across an interesting question on strings. Suppose you are given following:

Input string1: \"this is a test string\"
Input strin         


        

Answer 1

I've implemented it using Python3 at O(N) efficiency:

def get(s, alphabet="abc"):
    seen = {}
    for c in alphabet:
        seen[c] = 0
    seen[s[0]] = 1
    start = 0
    end = 0
    shortest_s = 0
    shortest_e = 99999
    while end + 1 < len(s):
        while seen[s[start]] > 1:
            seen[s[start]] -= 1
            start += 1
        # Constant time check:
        if sum(seen.values()) == len(alphabet) and all(v == 1 for v in seen.values()) and \
                shortest_e - shortest_s > end - start:
            shortest_s = start
            shortest_e = end
        end += 1
        seen[s[end]] += 1
    return s[shortest_s: shortest_e + 1]


print(get("abbcac")) # Expected to return "bca"