替换Objective-C中字符串中的一个字符

- (NSString *)replaceCharacterAtIndex:(NSUInteger)index withCharacter:(unichar)character {
    NSMutableString *mutableString = [self mutableCopy];
    [mutableString replaceCharactersInRange:NSMakeRange(index, 1) withString:[NSString stringWithCharacters:&character length:1]];
    return [mutableString copy];
}

NSString *originalString = @"Hello, World!";
NSString *newString = [originalString replaceCharacterAtIndex:6 withCharacter:'a'];
NSLog(@"%@", newString); // 输出 "Helloa, World!"