SPOJ962 Intergalactic Map（最大流）

题目问一张无向图能否从1点走到2点再走到3点，且一个点只走一次。

思维定势思维定势。。建图关键在于，源点向2点连边，1点和3点向汇点连边！

另外，题目数据听说有点问题，出现点大于n的数据。。

1 #include

2 #include

3 #include

4 #include

5 using namespace std;

6 #define INF (1<<30)

7 #define MAXN 666666

8 #define MAXM 666666

9

10 struct Edge{

11 int v,cap,flow,next;

12 }edge[MAXM];

13 int vs,vt,NE,NV;

14 int head[MAXN];

15

16 void addEdge(int u,int v,int cap){

17 edge[NE].v=v; edge[NE].cap=cap; edge[NE].flow=0;

18 edge[NE].next=head[u]; head[u]=NE++;

19 edge[NE].v=u; edge[NE].cap=0; edge[NE].flow=0;

20 edge[NE].next=head[v]; head[v]=NE++;

21 }

22

23 int level[MAXN];

24 int gap[MAXN];

25 void bfs(){

26 memset(level,-1,sizeof(level));

27 memset(gap,0,sizeof(gap));

28 level[vt]=0;

29 gap[level[vt]]++;

30 queue que;

31 que.push(vt);

32 while(!que.empty()){

33 int u=que.front(); que.pop();

34 for(int i=head[u]; i!=-1; i=edge[i].next){

35 int v=edge[i].v;

36 if(level[v]!=-1) continue;

37 level[v]=level[u]+1;

38 gap[level[v]]++;

39 que.push(v);

40 }

41 }

42 }

43

44 int pre[MAXN];

45 int cur[MAXN];

46 int ISAP(){

47 bfs();

48 memset(pre,-1,sizeof(pre));

49 memcpy(cur,head,sizeof(head));

50 int u=pre[vs]=vs,flow=0,aug=INF;

51 gap[0]=NV;

52 while(level[vs]

53 bool flag=false;

54 for(int &i=cur[u]; i!=-1; i=edge[i].next){

55 int v=edge[i].v;

56 if(edge[i].cap!=edge[i].flow && level[u]==level[v]+1){

57 flag=true;

58 pre[v]=u;

59 u=v;

60 //aug=(aug==-1?edge[i].cap:min(aug,edge[i].cap));

61 aug=min(aug,edge[i].cap-edge[i].flow);

62 if(v==vt){

63 flow+=aug;

64 for(u=pre[v]; v!=vs; v=u,u=pre[u]){

65 edge[cur[u]].flow+=aug;

66 edge[cur[u]^1].flow-=aug;

67 }

68 //aug=-1;

69 aug=INF;

70 }

71 break;

72 }

73 }

74 if(flag) continue;

75 int minlevel=NV;

76 for(int i=head[u]; i!=-1; i=edge[i].next){

77 int v=edge[i].v;

78 if(edge[i].cap!=edge[i].flow && level[v]

79 minlevel=level[v];

80 cur[u]=i;

81 }

82 }

83 if(--gap[level[u]]==0) break;

84 level[u]=minlevel+1;

85 gap[level[u]]++;

86 u=pre[u];

87 }

88 return flow;

89 }

90

91 int main(){

92 int t,n,m,a,b;

93 scanf("%d",&t);

94 while(t--){

95 scanf("%d%d",&n,&m);

96 memset(head,-1,sizeof(head));

97 vs=0; vt=n<<1|1; NV=vt+1; NE=0;

98 addEdge(vs,2,2); addEdge(1+n,vt,1); addEdge(3+n,vt,1);

99 for(int i=1; i<=n; ++i) addEdge(i,i+n,1);

100 addEdge(2,2+n,1);

101 while(m--){

102 scanf("%d%d",&a,&b);

103 if(a>n || b>n) continue;

104 addEdge(a+n,b,1); addEdge(b+n,a,1);

105 }

106 if(n<=2 || ISAP()<2) puts("NO");

107 else puts("YES");

108 }

109 return 0;

110 }

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