题目

#include<bits/stdc++.h>
using namespace std;
#define int long long
#define pb push_back
const int maxn = 2e5 + 5, maxm = 2e3 + 5;
int a[maxn];
int nxt;//下一个应该遍历的结点
int res = 0;
int n, m;
vector<int> G[maxn];
void dfs(int u){
    if(u == nxt) nxt++;
    for(int i = 0; i < G[u].size();){//这里不写i++
        int v = G[u][i];
        if(v < nxt){//之前遍历过的结点，跳过
            i++;
            continue;
        } 
        if(v > nxt){
            res++;
            dfs(nxt);//这里不i++, 等把nxt~v-1的结点遍历完再dfs(v)
            /*
            1
            4 1
            1 3
            */
        }
        else{
            dfs(v);
            i++;
        }
    }
}
void solve(){
	cin >> n >> m;
    for(int i = 1; i <= n; i++){
        G[i].clear();
    }
    res = 0;
    for(int i = 1; i <= m; i++){
        int u, v;
        cin >> u >> v;
//         if(u == v) continue;
//         if(u > v) swap(u, v);
        G[u].push_back(v);
        G[v].push_back(u);
    }
    for(int i = 1; i <= n; i++){
        sort(G[i].begin(), G[i].end());//按顺序来
    }
    nxt = 1;
    while(nxt <= n){
        res++;//向nxt连一条边
        dfs(nxt);
    }
    cout << res - 1 << '\n';//减去向1连的一条边
}
signed main(){
	ios::sync_with_stdio(0);
	cin.tie(0);
	int T;
	cin >> T;
	while(T--){
		solve();
	}
}