HDU 2767 Proving Equivalences 图论scc缩点

问一个图，最少需要加多少条边，使得这个图强联通。

Tarjan缩点，重建图，令a=入度为0的scc个数，b=出度为0的scc个数，ans=max(a,b)；

若图scc=1，本身强联通，ans=0；

 1 #include<bits/stdc++.h>
 2 using namespace std;
 3 const int MAXN = 20010;//点数
 4 const int MAXM = 200100;//边数
 5 struct Edge {
 6     int to,next;
 7 }edge[MAXM];
 8 int head[MAXN],tot,a,b;
 9 int Low[MAXN],DFN[MAXN],Stack[MAXN],Belong[MAXN];//Belong数组的值是1~scc
10 int Index,top; int scc;//强连通分量的个数
11 int in[MAXN],out[MAXN];
12 bool Instack[MAXN];
13 int num[MAXN];//各个强连通分量包含点的个数，数组编号1~scc //num数组不一定需要，结合实际情况
14
15 void addedge(int u,int v){
16     edge[tot].to = v;
17     edge[tot].next = head[u];
18     head[u] = tot++;
19 }
20 void Tarjan(int u){
21     int v;
22     Low[u] = DFN[u] = ++Index;
23     Stack[top++] = u;
24     Instack[u] = true;
25     for(int i = head[u];i != -1;i = edge[i].next) {
26         v = edge[i].to;
27         if( !DFN[v] ){
28             Tarjan(v);
29             if( Low[u] > Low[v] )
30                 Low[u] = Low[v];
31         }
32         else if(Instack[v] && Low[u] > DFN[v])
33             Low[u] = DFN[v];
34         }
35         if(Low[u] == DFN[u]){
36             scc++;
37             do{
38                 v = Stack[--top];
39                 Instack[v] = false;
40                 Belong[v] = scc;
41                 num[scc]++;
42             }while( v != u);
43         }
44 }
45 void find_degree(int u){
46     int v;
47     for(int i = head[u];i != -1;i = edge[i].next){
48         v = edge[i].to;
49         if(Belong[u]==Belong[v]) continue;
50         in[Belong[v]]++;
51         out[Belong[u]]++;
52     }
53 }
54 void solve(int N){
55     memset(DFN,0,sizeof(DFN));
56     memset(Instack,false,sizeof(Instack));
57     memset(num,0,sizeof(num));
58     Index = scc = top = 0;
59     for(int i = 1;i <= N;i++)
60         if(!DFN[i])
61         Tarjan(i);
62     for(int i=1;i<=N;i++){
63         find_degree(i);
64     }
65     if(scc==1) return;
66     for(int i=1;i<=scc;i++){
67         if(in[i]==0) a++;
68         if(out[i]==0) b++;
69     }
70 }
71 void init(){
72     tot = 0;
73     a=0,b=0;
74     memset(head,-1,sizeof(head));
75     memset(in,0,sizeof(in));
76     memset(out,0,sizeof(out));
77 }
78 int main(){
79     int n,m,u,v,T;
80     cin>>T;
81     while(T--){
82         scanf("%d%d",&n,&m);
83         init();
84         for(int i=1;i<=m;i++){
85             scanf("%d%d",&u,&v);
86             addedge(u,v);
87         }
88         solve(n);
89         cout<<max(a,b)<<endl;
90     }
91     return 0;
92 }