MergeSort with Objective-C

MergeSort

- (NSArray *)mergeSort:(NSArray *)array {
    
    if (array.count == 1) {
        return array;
    }
    
    // Split array in two
    NSInteger firstHalfCount = array.count/2;
    NSInteger secondHalfCount = array.count - firstHalfCount;
    NSArray *arrayOne = [array subarrayWithRange:NSMakeRange(0, firstHalfCount)];
    NSArray *arrayTwo = [array subarrayWithRange:NSMakeRange(firstHalfCount, secondHalfCount)];
    
    // Recursively split until we have one element in each
    NSArray *sortedFirst = [self mergeSort:arrayOne];
    NSArray *sortedSecond = [self mergeSort:arrayTwo];
    
    // Merge together
    NSArray *merged = [self mergeFirstArray:sortedFirst withSecondArray:sortedSecond];
    
    // Return merged
    return merged;
}

- (NSArray *)mergeFirstArray:(NSArray *)firstArray withSecondArray:(NSArray *)secondArray {
    
    NSMutableArray *mergedArray = [NSMutableArray array];
    
    NSInteger firstIndex = 0;
    NSInteger secondIndex = 0;
    
    // Merge elements
    while (firstIndex < firstArray.count && secondIndex < secondArray.count) {
        // Merge
        NSNumber *firstItem = firstArray[firstIndex];
        NSNumber *secondItem = secondArray[secondIndex];
        if (firstItem.integerValue < secondItem.integerValue) {
            [mergedArray addObject:firstItem];
            firstIndex++;
        } else {
            [mergedArray addObject:secondItem];
            secondIndex++;
        }
    }
    
    // Add any elements left over (they will already be sorted)
    while (firstIndex < firstArray.count) {
        [mergedArray addObject:firstArray[firstIndex]];
        firstIndex++;
    }

    // Add any elements left over (they will already be sorted)
    while (secondIndex < secondArray.count) {
        [mergedArray addObject:secondArray[secondIndex]];
        secondIndex++;
    }

    return mergedArray;
}

Assumes an array of NSNumber objects.