并查集
1.通俗易懂，适合初学者。
http://blog.****.net/dellaserss/article/details/7724401
2.讲解详细，有更多思考。
http://blog.****.net/dm_vincent/article/details/7655764
struct DisjointSet{
vector<int> father,rank;
DisjointSet(int n):father(n),rank(n){
for(int i=0; i<n; ++i)
father[i]=i;
}
int find(int v){
return father[v]=father[v]==v?v:find(father[v]);
}
void merge(int x,int y){
int a=find(x),b=find(y);
if(rank[a]<rank[b])
father[a]=b;
else{
father[b]=a;
if(rank[b]==rank[a])
++rank[a];
}
}
};