- (void)viewDidLoad {
    [super viewDidLoad];
    NSString *oneString = @"dfjkbnlskbanvjkfnb";
    NSString *twoString = @"vkjbnkdfdsfknbg ";
    NSLog(@"oneString====> %@ \n twoString====> %@ ",oneString,twoString);
    NSString *maxString = oneString;
    NSString *smallString = twoString;
    if (oneString.length < twoString.length) {
        maxString = twoString;
        smallString = oneString;
    }
    [self findMaxSubString:maxString smallString:smallString];
}
- (void)findMaxSubString:(NSString *)maxString smallString:(NSString *)smallString {
    NSString *printSubString = @"";
    for (int i = 0; i <smallString.length; i++) {
        NSString *str = [smallString substringWithRange:NSMakeRange(i, 1)];
        if ([maxString containsString:str] && ![printSubString containsString:str]) {
            printSubString = [printSubString stringByAppendingString:str];
        }
    }
    NSLog(@"printSubString====> %@",printSubString);
}