並查集

const int maxn = 10010;
int p[maxn];

int find(int x){if(x != p[x]) p[x] = find(p[x]); return p[x];}
int find(int x){return x == p[x] ? p[x] : p[x] = find(p[x]);}

for(int i = 0; i < n; ++i){
	p[i] = i;
}

find(x) == find(y);

p[find(x)] = find(y);