# 题目

“Damn Single (单身狗)” is the Chinese nickname for someone who is being single. You are supposed to find those who are alone in a big party, so they can be taken care of.

### Input Specification:

Each input file contains one test case. For each case, the first line gives a positive integer N (≤ 50,000), the total number of couples. Then N lines of the couples follow, each gives a couple of ID’s which are 5-digit numbers (i.e. from 00000 to 99999). After the list of couples, there is a positive integer M (≤ 10,000) followed by M ID’s of the party guests. The numbers are separated by spaces. It is guaranteed that nobody is having bigamous marriage (重婚) or dangling with more than one companion.

### Output Specification:

First print in a line the total number of lonely guests. Then in the next line, print their ID’s in increasing order. The numbers must be separated by exactly 1 space, and there must be no extra space at the end of the line.

# 题解

## 思路

• 这题真实迷惑
• 题目给出互为夫妻的两个人
• 给出参与派对的人
• 问你哪些人是单身的
• 迷惑的是，当一个人是已婚的，他对象没出现在party中，也当作单身处理

## 数据结构

• couple为一个哈希表，键和值互为一对couple
• ans 存放答案
• guests 是一个集合，存所有的宾客。（为了方便查找设置成了哈希集）

## 算法

• 将成对成对的情侣添加到couple里
• 遍历所有宾客
• 如果宾客不在couple里或者他的对象不在couple里
• 把它添加到ans里
• 对ans排序并输出

## 代码

• 由于使用Python能够AC，因此使用Python作题解。