hdu2767(利用tarjan将图变成强连通图)

题目
题意:求解将一个图变成强连通图需要添加的最小边数
思路:首先用tarjan将图缩点变成DAG,既然是强连通图,那么每个点都应该能进也能出,所以统计一下出度,入度为0的点的数量,要求边数最少,所以取max(innum,outnum)作为答案;

#include<bits/stdc++.h>
#include<map>
const int maxn = 3e4 + 10;
using namespace std;
int n,m,dfn[maxn],head[maxn],low[maxn],vis[maxn],stk[maxn],top,cnt,index,ans,scc,belong[maxn],in[maxn],out[maxn];
struct EDG {
    int v,nxt;
}edge[60000 + 10];
void Initial() {
    top = cnt = index = ans = scc = 0;
    memset(head,-1,sizeof(head));
    memset(vis,0,sizeof(vis));
    memset(dfn,0,sizeof(dfn));
    memset(belong,0,sizeof(belong));
    memset(out,0,sizeof(out));
    memset(in,0,sizeof(in));
}
void add_edge(int u,int v) {
    edge[cnt].v = v;
    edge[cnt].nxt = head[u];
    head[u] = cnt++;
}
void tarjan(int u) {
    dfn[u] = low[u] = ++index;
    stk[top++] = u;
    vis[u] = 1;
    int v;
    for(int i = head[u]; ~i; i=edge[i].nxt) {
        v = edge[i].v;
        if(!dfn[v]) {
            tarjan(v);
            low[u] = min(low[v],low[u]);
        }else if(vis[v]) {
            low[u] = min(low[u],dfn[v]);
        }
    }
    if(dfn[u] == low[u]) {
        scc++;
        do {
            v = stk[--top];
            belong[v] = scc;
            vis[v] = 0;
        }while(v != u);
    }
}
int main()
{
    int T;
    scanf("%d",&T);
    while(T--) {
        scanf("%d%d",&n,&m);
        Initial();
        for(int i = 1;i <= m; ++i) {
            int u,v;
            scanf("%d%d",&u,&v);
            add_edge(u,v);
        }
        for(int i = 1;i <= n; ++i) {
            if(!dfn[i]) {
                tarjan(i);
            }
        }
        for(int i = 1;i <= n; i++) {
            for(int j = head[i];~j;j = edge[j].nxt) {
                int v = edge[j].v;
                if(belong[i] != belong[v]) {
                    out[belong[i]]++;
                    in[belong[v]]++;
                }
            }
        }
        int inn = 0,outn = 0;
        for(int i = 1;i <= scc; ++i) {
            if(in[i] == 0) inn++;
            if(out[i] == 0) outn++;
        }
        if(scc == 1) cout<<0<<endl;
        else cout<<max(inn,outn)<<endl;
    }
    return 0;
}
hdu2767(利用tarjan将图变成强连通图)hdu2767(利用tarjan将图变成强连通图) Alstein 发布了42 篇原创文章 · 获赞 27 · 访问量 7617 私信 关注
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