quick select, find nth largest number

QuickSelect.m

-(void) swapFirst:(int)first withSecond:(int)second inArray:(NSMutableArray *)array
{
    NSNumber *temp = array[first];
    array[first] = array[second];
    array[second] = temp;
    return;
}

-(int) findKthLargestNumber:(int)k
                    inArray:(NSMutableArray *)array
                      begin:(int)begin
                        end:(int)end
{
    int pivot = (begin + end) / 2;
    [self swapFirst:pivot withSecond:end inArray:array];
    int pivotPosition = begin;
    for (int i = begin; i < end; ++i) {
        if ([array[i] intValue] < [array[end] intValue]) {
            [self swapFirst:i withSecond:pivotPosition inArray:array];
            pivotPosition++;
        }
    }
    [self swapFirst:pivotPosition withSecond:end inArray:array];
    if (pivotPosition == k - 1) {
        return [array[pivotPosition] intValue];
    } else if (pivotPosition < k - 1) {
        return [self findKthLargestNumber:k inArray:array begin:pivotPosition + 1 end:end];
    } else {
        return [self findKthLargestNumber:k inArray:array begin:begin end:pivotPosition - 1];
    }
}