Algorithm to determine if array contains n…n+m?

Question

I saw this question on Reddit, and there were no positive solutions presented, and I thought it would be a perfect question to ask here. This was in a thread about interview

Answer 1

def test(a, n, m):
    seen = [False] * m
    for x in a:
        if x < n or x >= n+m:
            return False
        if seen[x-n]:
            return False
        seen[x-n] = True
    return False not in seen

print test([2, 3, 1], 1, 3)
print test([1, 3, 1], 1, 3)
print test([1, 2, 4], 1, 3)

Note that this only makes one pass through the first array, not considering the linear search involved in not in. :)

I also could have used a python set, but I opted for the straightforward solution where the performance characteristics of set need not be considered.

Update: Smashery pointed out that I had misparsed "constant amount of memory" and this solution doesn't actually solve the problem.