并查集

void init()
{
    for(int i = 1;i <= n;i++)    pre[i] = i;
} 

int findd(int x)
{
    int root = x;
    while(pre[root] != root)    root = pre[root];
    int i = x,j;
    while(pre[i] != root)
    {
        j = pre[i];
        pre[i] = root;
        i = j;
    }
    return root;
}
void join(int a,int b)
{
    int x = findd(a),y = findd(b);
    if(x != y)  pre[x] = y;
}

int findd(int x)                     
{
    if (x != pre[x])
    {
       return pre[x] = findd(pre[x]);
    }
    return x;
}