gistfile1.txt

- (instancetype)add:(NSString *)num {
    
    int sLength = (int)MIN(num.length, self.num.length);
    int length1 = (int)self.num.length;
    int length2 = (int)num.length;
    int lengthDiff = abs(length1 - length2);

    BOOL larger = num.length > self.num.length;
    NSNumber *toAdd;
    NSNumberFormatter *numberFormatter = [[NSNumberFormatter alloc] init];
    NSString *newString = @"";
    for (int i = (sLength - 1); i >= 0 ; i--) {

        NSInteger c1Index = larger? i:i + lengthDiff;
        NSString *c1 = [self.num substringWithRange:NSMakeRange(c1Index, 1)];
        
        NSInteger c2Index = larger? i+lengthDiff : i;
        NSString *c2 = [num substringWithRange:NSMakeRange(c2Index, 1)];
       
        NSNumber *n1 = [numberFormatter numberFromString:c1];
        NSNumber *n2 = [numberFormatter numberFromString:c2];
        int n3 = n1.intValue + n2.intValue + toAdd.intValue;
        toAdd = [NSNumber numberWithInt:(int)n3/10];
        NSNumber * rest = [NSNumber numberWithInt:fmod(n3, 10)];
       
        newString = [[rest stringValue] stringByAppendingString:newString];
    }
    if (lengthDiff > 0) {
        if (larger) {
            self.num = [[num substringWithRange:NSMakeRange(0, lengthDiff)] stringByAppendingString:newString];
        } else {
            self.num = [[self.num substringWithRange:NSMakeRange(0, lengthDiff)] stringByAppendingString:newString];
        }
    }
    return self;
}