How Many Tables（并查集）

题目：
Today is Ignatius’ birthday. He invites a lot of friends. Now it’s dinner time. Ignatius wants to know how many tables he needs at least. You have to notice that not all the friends know each other, and all the friends do not want to stay with strangers.

One important rule for this problem is that if I tell you A knows B, and B knows C, that means A, B, C know each other, so they can stay in one table.

For example: If I tell you A knows B, B knows C, and D knows E, so A, B, C can stay in one table, and D, E have to stay in the other one. So Ignatius needs 2 tables at least.
Input
The input starts with an integer T(1<=T<=25) which indicate the number of test cases. Then T test cases follow. Each test case starts with two integers N and M(1<=N,M<=1000). N indicates the number of friends, the friends are marked from 1 to N. Then M lines follow. Each line consists of two integers A and B(A!=B), that means friend A and friend B know each other. There will be a blank line between two cases.
Output
For each test case, just output how many tables Ignatius needs at least. Do NOT print any blanks.
Sample Input
2
5 3
1 2
2 3
4 5

5 1
2 5
Sample Output
2
4

题意：
认识的人才坐在一起，问需要多少桌子。

代码：

#include<stdio.h>
#include<stdlib.h>
#include<math.h>
#include<string.h>
#include<algorithm>
#include<iostream>
#include<queue>
#include<stack>
#include<cstdio>
#include<cstring>
#include<vector>
#include<cmath>
#include<map>
#define exp 1e-9
#define PI acos(-1.0)
#define mod 100000
#define INF 0x3f3f3f3f
using namespace std;
typedef long long LL;
int pre[1005];

int finds(int k)
{
    if(pre[k]==k)return k;
    return pre[k]=finds(pre[k]);
}

void join(int x,int y)
{

    int fx=finds(x),fy=finds(y);
    if(fx!=fy)  pre[fx ]=fy;
}

int main()
{
    int t,n,m,ans;
    scanf("%d",&t);
    while(t--)
    {
        scanf("%d%d",&n,&m);
        ans=n;
        for(int i=1;i<=n;i++)
            pre[i]=i;
        for(int i=1;i<=m;i++)
        {
            int a,b;
            scanf("%d%d",&a,&b);
            if(finds(a)!=finds(b))
            {
                join(a,b);
                ans--;
            }
        }
        printf("%d\n",ans);
    }
    return 0;
}