Question

I strugle with this for second day…
I have an NSArray of ints, wrapped as NSNumbers. I need to find the lowest positive int, that is not present in the array.

Answer 1

You can use an NSMutableIndexSet as an efficient way to store a set of integers. Just iterate over the array, stuffing them in the NSMutableIndexSet, and then when you’re done, you can walk up the index set until you find a hole. Here’s an example:

int lowestPositiveIntNotInArray(NSArray *array) {
    NSMutableIndexSet *set = [NSMutableIndexSet indexSet];
    for (NSNumber *num in array) {
        [set addIndex:[num intValue]];
    }
    // assuming 0 is a valid result here
    NSUInteger seed = [set firstIndex];
    if (seed > 0) {
        return 0;
    }
    NSUInteger next;
    while ((next = [set indexGreaterThanIndex:seed]) != NSNotFound) {
        if (next - seed > 1) {
            // there's a hole here
            break;
        }
        seed = next;
    }
    return seed+1;
}