並查集：

#include <cstdio>
using namespace std;
const int cityNum = 1000 ;
int city[cityNum];
int N,M;
int total;

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

int findSet(int x){ 
    int tmp = x;
    do{
        tmp = city[tmp];
    }while(tmp != city[tmp]);
    city[x] 
 
= tmp;
    return tmp;
}

void unionSet(int x,int y){//Y->X
    int xP = findSet(x);
    int yP = findSet(y);
    if(xP != yP){
        total--;
        city[yP] = xP;
        city[y] = xP; 
    }
}

int main(){
    int city1,city2;
    while(scanf("%d %d",&N,&M) && N){
        total 
 
= N-1;
        init();
        for(int i=0;i<M;i++){
            scanf("%d %d",&city1,&city2);
            unionSet(city1,city2);
        }
        printf("%d\n",total);
    }
    return 0;
}