poj 2524 Ubiquitous Religions

#include <stdio.h>

int father[500005], num[500005];

void MakeSet(int n)
{
    for(int i = 1; i<=n; i++)
    {
        father[i] = i;
        num[i] = 0;
    }
}

int FindSet(int x)
{
    if(x != father[x])
    {
        father[x] = FindSet(father[x]);
    }

    return father[x];
}

void UnionSet(int a, int b)
{
    int x = FindSet(a);
    int y = FindSet(b);

    if(x == y)
        return ;
    if(num[x] >= num[y])
    {
        father[y] = x;
        num[x] += 1;
    }
    else
    {
        father[x] = y;
        num[y] += 1;
    }
}

int main()
{
    int n, m, k = 1;
    int a, b, sum;
    while(scanf("%d%d", &n, &m)!=EOF && n && m)
    {
        MakeSet(n);
        while(m--)
        {
            scanf("%d%d", &a, &b);
            UnionSet(a, b);
        }
        sum = 0;
        for(int i = 1; i<=n; i++)
            sum += num[i];
        printf("Case %d: %d\n", k++, n-sum);
    }
}