//NSMutableString stringWithCapacity, stringWithString
        NSMutableString *str1 = [NSMutableString stringWithCapacity:100];
        NSMutableString *str2 = [NSMutableString stringWithString:@"Hello, 上海!"];
        NSLog(@"str1 address: %p", str1);
        NSLog(@"str2: %@", str2);

//NSMutableString appendFormat, appendString
        [str1 appendFormat:@"苹果的iO"];
        [str1 appendFormat:@"S, 很喜欢best!"];
        NSLog(@"str1 appendFormat: %@", str1);
        [str1 appendString:@"你呢？"];
        NSLog(@"str1 appendString: %@", str1);

//NSMutableString deleteCharactersInRange
        [str1 deleteCharactersInRange: [str1 rangeOfString:@"best"]];
        NSLog(@"str1 deleteCharactersInRange: %@", str1);

//NSMutableString 
        [str1 insertString:@"系统" atIndex:(6)];
        NSLog(@"str1 insertString:atIndex: %@", str1);

//NSMutableString assign/address
        NSMutableString *str3 = str1;
        NSLog(@"str1 address: %p", str1);
        NSLog(@"str3 address: %p", str3);
        NSLog(@"str3: %@", str3);