HDU 4635 —— Strongly connected——————【 强连通、最多加多少边仍不强连通】

#include<stdio.h>
#include<algorithm>
#include<queue>
#include<stack>
#include<string.h>
#include<vector>
using namespace std;
typedef long long INT;
const int maxn = 100100;
struct Edge{
    int from,to,dist,next;
    Edge(){}
    Edge(int _to,int _next):to(_to),next(_next){}
}edges[maxn];
int tot, head[maxn];
void init(){
    tot = 0;
    memset(head,-1,sizeof(head));
}
void AddEdge(int _u,int _v){
    edges[tot] = Edge(_v,head[_u]);
    head[_u] = tot++;
}
int dfs_clock, scc_cnt;
int sccno[maxn], dfn[maxn], lowlink[maxn];
stack<int>S;
void dfs(int u){
    lowlink[u] = dfn[u] = ++dfs_clock;
    S.push(u);
    for(int i = head[u]; i != -1; i = edges[i].next){
        int v = edges[i].to;
        if(!dfn[v]){
            dfs(v);
            lowlink[u] = min(lowlink[u], lowlink[v]);
        }else if(!sccno[v]){
            lowlink[u] = min(lowlink[u],dfn[v]);
        }
    }
    if(lowlink[u] == dfn[u]){
        scc_cnt++;
        for(;;){
            int x = S.top();
            S.pop();
            sccno[x] = scc_cnt;
            if(x == u){
                break;
            }
        }
    }
}
void find_scc(int n){
    dfs_clock = scc_cnt = 0;
    memset(sccno,0,sizeof(sccno));
    memset(dfn,0,sizeof(dfn));
    for(int i = 1; i <= n; i++){
        if(!dfn[i]){
            dfs(i);
        }
    }
}
int outdeg[maxn], indeg[maxn];
INT sccsz[maxn];
int main(){
    int T, m, cas = 0;
    INT n;
    scanf("%d",&T);
    while(T--){
        scanf("%lld%d",&n,&m);
        init();
        int a,b;
        for(int i = 0; i < m; i++){
            scanf("%d%d",&a,&b);
            AddEdge(a,b);
        }
        find_scc(n);
        printf("Case %d: ",++cas);
        if(scc_cnt == 1){
            puts("-1");continue;
        }
        memset(indeg,0,sizeof(indeg));
        memset(outdeg,0,sizeof(outdeg));
        memset(sccsz,0,sizeof(sccsz));
        for(int i = 1; i <= n; i++){
            sccsz[sccno[i]]++;
            for(int j = head[i]; j != -1; j = edges[j].next){
                int v = edges[j].to;
                if(sccno[i] == sccno[v]){
                    continue;
                }
                indeg[sccno[v]]++;
                outdeg[sccno[i]]++;
            }
        }
        INT ans = 0;
        for(int i = 1; i <= scc_cnt; i++){
            if(indeg[i] == 0 ||outdeg[i] == 0){
                ans = max( ans, (n*n - n - sccsz[i]*(n - sccsz[i]))-m );
            }
        }
        printf("%lld\n",ans);
    }
    return 0;
}

/*
555
7 9
1 5
1 2
5 6
6 7
7 5
6 4
2 4
4 3
3 2

*/