iOS 自动将数字三位一分割

+(NSString *)ChangeNumberFormat:(NSString *)num
{
    if (num != nil) {
        return @"";
    }
    int count = 0;
    long long int a = num.longLongValue;
    while (a != 0)
    {
        count++;
        a /= 10;
    }
    NSMutableString *string = [NSMutableString stringWithString:num];
    NSMutableString *newstring = [NSMutableString string];
    while (count > 3) {
        count -= 3;
        NSRange rang = NSMakeRange(string.length - 3, 3);
        NSString *str = [string substringWithRange:rang];
        [newstring insertString:str atIndex:0];
        [newstring insertString:@"," atIndex:0];
        [string deleteCharactersInRange:rang];
    }
    [newstring insertString:string atIndex:0];
    return newstring;
}