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poj1637 判断混合图是否能形成欧拉回路：网络流做奇偶性判断（inspire）

  1 //dinic算法：

  2 #include <stdio.h>

  3 #include <string.h>

  4 #include <math.h>

  5 #include <iostream>

  6 #include <algorithm>

  7

  8 #define INF 0x3f3f3f3f

  9 using namespace std;

 10 const int maxn = 2010;

 11 const int maxm = 19010;

 12

 13 int e,S,T;

 14 int first[maxn],next[maxm],work[maxm];

 15 int u[maxm],v[maxm],w[maxm];

 16

 17 void init(){

 18     e = 0;

 19     memset(first,-1,sizeof(first));

 20 }

 21

 22 void addedge(int a,int b,int c){

 23     v[e] = b;next[e] = first[a];w[e] = c;first[a] = e++;

 24     v[e] = a;next[e] = first[b];w[e] = 0;first[b] = e++;

 25 }

 26 int d[maxn],q[maxn];

 27 int bfs(){

 28     int rear = 0;

 29     memset(d,-1,sizeof(d));

 30     d[S] = 0;q[rear++] = S;

 31     for(int i = 0;i < rear;i++){

 32         for(int j = first[q[i]];j != -1;j = next[j])

 33             if(w[j] && d[v[j]] == -1){

 34                 d[v[j]] = d[q[i]] + 1;

 35                 q[rear++] = v[j];

 36                 if(v[j] == T)   return 1;

 37             }

 38     }

 39     return 0;

 40 }

 41

 42 int dfs(int cur,int a){

 43     if(cur == T)    return a;

 44     for(int &i = work[cur];i != -1;i = next[i]){

 45         if(w[i] && d[v[i]] == d[cur] + 1)

 46             if(int t = dfs(v[i],min(a,w[i]))){

 47                 w[i] -= t;w[i^1] += t;

 48                 return t;

 49             }

 50     }

 51     return 0;

 52 }

 53

 54 int dinic(){

 55     int ans = 0;

 56     while(bfs()){

 57         memcpy(work,first,sizeof(first));

 58         while(int t = dfs(S,INF))   ans += t;

 59     }

 60     return ans;

 61 }

 62 int M,N,CASE;

 63 int ind[maxn],outd[maxn];

 64 int main(){

 65     scanf("%d",&CASE);

 66     while(CASE--)

 67     {

 68         scanf("%d%d",&N,&M);

 69         init();

 70         S=N+1;T=N+2;

 71         memset(ind,0,sizeof(ind));

 72         memset(outd,0,sizeof(outd));

 73         for(int i=1;i<=M;i++){

 74             int x,y,c;

 75             scanf("%d%d%d",&x,&y,&c);

 76             if (c==1) outd[x]++,ind[y]++;

 77             if (c!=1) {

 78                 outd[x]++,ind[y]++;

 79                 addedge(x,y,1);

 80             }

 81         }

 82         bool isok=true;

 83         for(int i=1;i<=N;i++) if ((outd[i]-ind[i])%2!=0) isok=false;

 84         if (!isok) {

 85             printf("impossible\n");

 86             continue;

 87         }

 88         int sum=0;

 89         for(int i=1;i<=N;i++){

 90             int k=(outd[i]-ind[i])/2;

 91             if (k<0) addedge(i,T,-k);

 92             if (k>0) {

 93                 sum+=k;

 94                 addedge(S,i,k);

 95             }

 96         }

 97         int flow=dinic();

 98 ////        cout<<sum<<","<<flow<<endl;

 99         if (flow==sum) printf("possible\n");else printf("impossible\n");

100     }

101     return 0;

102 }

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