OC冒泡排序

NSMutableArray *p = [[NSMutableArray alloc] initWithObjects:@"3",@"5",@"4",@"1",@"7",@"6",@"4",nil];
    for (int i = 0; i<[p count]; i++)
    {
        for (int j=i+1; j<[p count]; j++)
        {
            int a = [[p objectAtIndex:i] intValue];
            int b = [[p objectAtIndex:j] intValue];
            if (a > b)
            {
                [p replaceObjectAtIndex:i withObject:[NSString stringWithFormat:@"%d",b]];
                [p replaceObjectAtIndex:j withObject:[NSString stringWithFormat:@"%d",a]];
            }
        }
    }
    for (int i = 0; i<[p count]; i++)
    {
        NSLog(@"%@",[p objectAtIndex:i]);
    }