[FIXED] What's wrong with this solution for Max Counters codility challenge

Issue

So I've been going through the tests on codility and got bit stuck with the "Max Counters" one (link https://codility.com/demo/take-sample-test/max_counters). My first, and obvious solution was the following one:

def solution(N, A):

    counters = N * [0];    

    for a in A:
        if 1 <= a <= N:
            counters[a - 1] += 1;
        elif a == N + 1:
            counters = N * [max(counters)];

    return counters

which works just fine, but takes too much time, due to the fact that each call to max counters fills an entire array.

So I came up with the following solution which seems to work ok for small inputs, but randomly provides incorrect results for medium and large ones.

def solution(N, A):

    counters = N * [0];
    current_max = 0;
    last_update = 0;

    for a in A:
        if 1 <= a <= N:
            counters[a - 1] += 1;

            if counters[a - 1] < last_update:
                counters[a - 1] = last_update + 1;

            if counters[a - 1] > current_max:
                current_max = counters[a - 1];

        elif a == N + 1:
            last_update = current_max;

    for i in xrange(len(counters)):
        if counters[i] < last_update:
            counters[i] = last_update;           

    return counters

I can't seem to figure out what's wrong with it.

Edit: Result - http://codility.com/demo/results/demoQA7BVQ-NQT/

Solution

One problem is here:

counters[a - 1] += 1
if counters[a - 1] < last_update:
    counters[a - 1] = last_update + 1

what if counters[a - 1] was last_update - 1?

Answered By - jonrsharpe