直接上代码

//调用返回结果
NSArray *arr = [self findSequentialStrings:@"dcbabcdcba12356543" subStrMinLen:4];
NSLog(@"%@",arr);

//MARK: - 传入一组字符串originalStr，和子字符串长度subStrMinLen，返回查找的子字符串数组
- (NSArray *)findSequentialStrings:(NSString *)originalStr subStrMinLen:(NSInteger)subStrMinLen
{
    if (originalStr.length < subStrMinLen) return nil;
    
    NSInteger i = 0;
    NSMutableString *appendStr = nil;
    BOOL isNeedRefresh = NO;
    NSMutableArray *subStrArr = @[].mutableCopy;
    for (; i < originalStr.length;) {
        if (!isNeedRefresh && [self findSingleSequentialSubStrWithStartIndex:i originalStr:originalStr subStrMinLen:subStrMinLen]) {
            if (!appendStr) {
                appendStr = [NSMutableString string];
            }
            [appendStr appendString:[originalStr substringWithRange:NSMakeRange(i, subStrMinLen)]];
            i += subStrMinLen;
            isNeedRefresh = YES;
            if (i == originalStr.length) {
                [subStrArr addObject:appendStr.mutableCopy];
            }
        }else{
            if (appendStr.length == 0) {
                isNeedRefresh = NO;
                ++ i;
            }else{
                char c1 = [originalStr characterAtIndex:i-1];
                NSNumber *cNumber1 = [NSNumber numberWithChar:c1];
                char c2 = [originalStr characterAtIndex:i];
                NSNumber *cNumber2 = [NSNumber numberWithChar:c2];
                
                char startChar = [appendStr characterAtIndex:0];
                char oChar = [appendStr characterAtIndex:1];
                NSInteger diffNum = [[NSNumber numberWithChar:oChar] integerValue] - [[NSNumber numberWithChar:startChar] integerValue];
                
                if (diffNum > 0) {
                    if ([cNumber2 integerValue] - [cNumber1 integerValue] == 1) {
                        [appendStr appendString:[originalStr substringWithRange:NSMakeRange(i, 1)]];
                        isNeedRefresh = !(i == originalStr.length - 1);
                        ++ i;
                    }else{
                        isNeedRefresh = NO;
                    }
                }else if (diffNum < 0) {
                    if ([cNumber2 integerValue] - [cNumber1 integerValue] == -1) {
                        [appendStr appendString:[originalStr substringWithRange:NSMakeRange(i, 1)]];
                        isNeedRefresh = !(i == originalStr.length - 1);
                        ++ i;
                    }else{
                        isNeedRefresh = NO;
                    }
                }
                if (appendStr.length > 0 && isNeedRefresh == NO) {
                    [subStrArr addObject:appendStr];
                    appendStr = nil;
                }
            }
        }
    }
    if (subStrArr.count == 0) {
        return nil;
    }
    return subStrArr.copy;
}

#pragma mark - private method
//MARK: - 根据起始位置startIndex和要求的子字符串长度subStrMinLen，判断是否满足要求
- (BOOL)findSingleSequentialSubStrWithStartIndex:(NSInteger)startIndex originalStr:(NSString *)originalStr subStrMinLen:(NSInteger)subStrMinLen
{
    if (startIndex + subStrMinLen > originalStr.length) {
        return NO;
    }
    //起始位置的字符
    char startChar = [originalStr characterAtIndex:startIndex];
    //
    char minLenChar = [originalStr characterAtIndex:startIndex+subStrMinLen-1];
    //判断下标startIndex+subStrLen对应的字符的ASCII值与下标startIndex对应的ASCII值相差是否为subStrLen，相等再继续比较中间子字符串是否连续
    NSInteger minLenCharValue = [[NSNumber numberWithChar:minLenChar] integerValue];
    NSInteger startCharValue = [[NSNumber numberWithChar:startChar] integerValue];
    NSInteger diffNum = minLenCharValue - startCharValue;
    if (diffNum == subStrMinLen-1 || diffNum == -(subStrMinLen-1)) {
        if (abs((int)diffNum) == 1) {
            return YES;
        }
        if ([self isSubSectionSequential:[originalStr substringWithRange:NSMakeRange(startIndex, subStrMinLen-2)] diffNum:diffNum]) {
            //连续的递增或递减
            return YES;
        }
    }
    return NO;
}

//MARK: - 判断区间[startIndex, startIndex+subStrMinLen-2]是否是连续的
- (BOOL)isSubSectionSequential:(NSString *)subStr diffNum:(NSInteger)diffNum
{
    BOOL isSequential = YES;
    for (NSInteger i = 0; i < subStr.length - 1; i++) {
        char c1 = [subStr characterAtIndex:i];
        NSNumber *cNumber1 = [NSNumber numberWithChar:c1];
        char c2 = [subStr characterAtIndex:i+1];
        NSNumber *cNumber2 = [NSNumber numberWithChar:c2];
        if (diffNum > 0) {
            if ([cNumber2 integerValue] - [cNumber1 integerValue] != 1) {
                isSequential = NO;
            }
        }else{
            if ([cNumber2 integerValue] - [cNumber1 integerValue] != -1) {
                return NO;
            }
        }
    }
    return isSequential;
}

打印结果：

(
dcba,
dcba,
6543
)