1.获取到某个字符串中所有的子字符串

-(NSSet *)GetAllSubStrsWithString:(NSString *)aString
{
    NSMutableSet *set = [NSMutableSet set];
    for (NSUInteger m = 0 ; m < aString.length - 1; m ++)
    {
        for (NSUInteger n = 1 ; n <  aString.length - m; n ++)
        {
            NSRange range = NSMakeRange(m, n);
            NSString *subString = [aString substringWithRange:range];
            [set addObject:subString];
        }
    }
    return  set;
}

2.算法

-(NSString *)getResultWithStirng1:(NSString *)string1
                          String2:(NSString *)string2;
{
    
    NSUInteger maxLegth = MAX(string1.length, string2.length);
    NSString *maxString = string1.length == maxLegth ? string1 : string2;
    NSString *minString = [string1 isEqualToString:maxString] ? string2 : string1;
    if ([maxString containsString:minString])
    {
        return minString;
    }
    NSMutableArray <NSString *>*resultAry = [NSMutableArray array];
    
    NSSet <NSString *>*SubStringset = [self GetAllSubStrsWithString:minString];
    [SubStringset enumerateObjectsUsingBlock:^(NSString * subString, BOOL * _Nonnull stop) {
        if ([maxString containsString:subString])
        {
            [resultAry addObject:subString];
        }
    }];
    
    [resultAry sortUsingComparator:^NSComparisonResult(NSString *result1, NSString *result2) {
        return result1.length > result2.length;
    }];
    NSString *result = resultAry.lastObject;
    return result;
}

3、测试

NSString *string1 = @"01dsadasldas2daasdasdadditdddiodsad01dsadasldas2additdddiodsad";
    NSString *string2 = @"0ffdlvdpvdk12addiwwtiodsdas01dsadasldas2additdddiodsad";
    
    NSString *result = [self getResultWithStirng1:string2 String2:string1];
    
    NSLog(@"%@",result);