get_products_of_all_ints_except_at_index.m

NSArray * bruteForceProduct(NSArray *array) {
    
    NSMutableArray *outputArray = [NSMutableArray arrayWithCapacity:[array count]];
    for (NSInteger i = 0 ; i < [array count]; i++) {
        NSInteger currentNumber = 1;
        for (NSInteger j = 0 ; j < [array count] ; j++) {
            if ( i != j ) {
                currentNumber *= [array[j] integerValue];
            }
        }
        [outputArray addObject:@(currentNumber)];
    }
    
    return [NSArray arrayWithArray:outputArray];
}

NSArray * cleverProduct(NSArray *array) {
    NSMutableArray *productArray = [NSMutableArray arrayWithCapacity:[array count]];
    
    for (NSInteger i = 0; i < [array count]; i++) {
        [productArray addObject:@1];
    }
    
    NSInteger product = 1;
    
    for (NSInteger i = 0; i < [productArray count]; i++) {
        productArray[i] = @(product);
        product *= [array[i] integerValue];
    }
    
    product = 1;
    
    for (NSInteger i = [productArray count] - 1; i >= 0; i--) {
        productArray[i] = @([productArray[i] integerValue] * product);
        product *= [array[i] integerValue];
    }
    
    return [NSArray arrayWithArray:productArray];
}