NSArray和NSMutableString，枚举和排序

    NSArray *array = @[@"string 1",
        @"string 3",
        @"string 4",
        @"string 2"
    ];
    //使用block，枚举所有数组元素，相当于forin循环
    NSLog(@"排序之前：\n");
    [array enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        NSLog(@"object at index %d = %@\n", idx, obj);
    }];
    
    NSLog(@"排序: \n");
    
    //注意NSComparisonResult的用法
    NSMutableArray *sortArray = [NSMutableArray arrayWithArray:array];
    //sortUsingComparator方法只可以直接改变NSMutableArray元素顺序
    [sortArray sortUsingComparator:^NSComparisonResult(id obj1, id obj2) {
        NSString *str1 = obj1;
        NSString *str2 = obj2;
        return [str1 compare:str2];
    }];
    
    [sortArray enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        NSLog(@"object at index %d = %@\n", idx, obj);
    }];
    
    //NSArray是只读的，只能使用sortedArrayUsingComparator，并重新赋值
    array = [array sortedArrayUsingComparator:^NSComparisonResult(id obj1, id obj2) {
        NSString *str1 = obj1;
        NSString *str2 = obj2;
        return [str1 compare:str2];
    }];
    
    [array enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        NSLog(@"object at index %d = %@\n", idx, obj);
    }];

NSDictionary、NSMutableDictionary也有类似

NSSet与NSArray的区别，在于同一个对象只允许添加一次，而NSArray没有这个限制