remove common letters in two Strings ---- iOS SDK

NSString *combined = [string1 stringByAppendingString:string2];







NSMutableString *result = [combined mutableCopy];

NSMutableSet *chars = [NSMutableSet set];

[result enumerateSubstringsInRange:NSMakeRange(0, [result length])

               options:NSStringEnumerationByComposedCharacterSequences

            usingBlock:^(NSString *substring, NSRange substringRange, NSRange enclosingRange, BOOL *stop) {

                if ([chars containsObject:substring]) {

                    [result deleteCharactersInRange:substringRange];

                } else {

                    [chars addObject:substring];

                }

            }];