loj 1156(二分+最大流）
题目链接：http://acm.hust.edu.cn/vjudge/problem/viewProblem.action?id=26870
思路：由于溢出问题，wa了半天，还以为构图错了呢，查了半天！一看到最大最小问题，就应该想到二分，二分最大距离，然后建图跑最大流验证。
1 #include<iostream> 2 #include<cstdio> 3 #include<cstring> 4 #include<algorithm> 5 #include<queue> 6 using namespace std; 7 #define MAXN 222 8 #define MAXM 222222 9 #define inf 1<<30 10 #define FILL(a,b) memset(a,b,sizeof(a)) 11 12 struct Edge{ 13 int v,cap,next; 14 }edge[MAXM]; 15 16 int n,D,vs,vt,NV,NE; 17 int head[MAXN]; 18 19 void Insert(int u,int v,int cap) 20 { 21 edge[NE].v=v; 22 edge[NE].cap=cap; 23 edge[NE].next=head[u]; 24 head[u]=NE++; 25 26 edge[NE].v=u; 27 edge[NE].cap=0; 28 edge[NE].next=head[v]; 29 head[v]=NE++; 30 } 31 32 int level[MAXN],gap[MAXN]; 33 void bfs(int vt) 34 { 35 FILL(level,-1); 36 FILL(gap,0); 37 queue<int>que; 38 que.push(vt); 39 level[vt]=0; 40 gap[0]++; 41 while(!que.empty()){ 42 int u=que.front(); 43 que.pop(); 44 for(int i=head[u];i!=-1;i=edge[i].next){ 45 int v=edge[i].v; 46 if(level[v]!=-1)continue; 47 level[v]=level[u]+1; 48 gap[level[v]]++; 49 que.push(v); 50 } 51 } 52 } 53 54 int pre[MAXN],cur[MAXN]; 55 int SAP(int vs,int vt) 56 { 57 bfs(vt); 58 memcpy(cur,head,sizeof(head)); 59 int u=pre[vs]=vs,aug=inf,maxflow=0; 60 gap[0]=NV; 61 while(level[vs]<NV){ 62 bool flag=false; 63 for(int &i=cur[u];i!=-1;i=edge[i].next){ 64 int v=edge[i].v; 65 if(edge[i].cap>0&&level[u]==level[v]+1){ 66 flag=true; 67 aug=min(aug,edge[i].cap); 68 pre[v]=u; 69 u=v; 70 if(v==vt){ 71 maxflow+=aug; 72 for(u=pre[u];v!=vs;v=u,u=pre[u]){ 73 edge[cur[u]].cap-=aug; 74 edge[cur[u]^1].cap+=aug; 75 } 76 aug=inf; 77 } 78 break; 79 } 80 } 81 if(flag)continue; 82 int minlevel=NV; 83 for(int i=head[u];i!=-1;i=edge[i].next){ 84 int v=edge[i].v; 85 if(edge[i].cap>0&&level[v]<minlevel){ 86 minlevel=level[v]; 87 cur[u]=i; 88 } 89 } 90 if(--gap[level[u]]==0)break; 91 level[u]=minlevel+1; 92 gap[level[u]]++; 93 u=pre[u]; 94 } 95 return maxflow; 96 } 97 98 struct Node{ 99 int d; 100 char flag; 101 }node[MAXN]; 102 103 void Build(int limit) 104 { 105 NE=0; 106 vs=0,vt=2*n+1,NV=vt+1; 107 FILL(head,-1); 108 for(int i=1;i<=n;i++){ 109 if(node[i].flag=='B')Insert(i,i+n,2); 110 else if(node[i].flag=='S')Insert(i,i+n,1); 111 } 112 for(int i=1;i<=n;i++){ 113 for(int j=i+1;j<=n;j++){ 114 if(node[j].d-node[i].d<=limit){ 115 Insert(i+n,j,2); 116 } 117 } 118 } 119 for(int i=1;i<=n;i++){ 120 if(node[i].d<=limit)Insert(vs,i,2); 121 if(D-node[i].d<=limit)Insert(i+n,vt,2); 122 } 123 if(D<=limit)Insert(vs,vt,2); 124 } 125 126 int main() 127 { 128 int _case,t=1; 129 char str[222]; 130 scanf("%d",&_case); 131 while(_case--){ 132 scanf("%d%d",&n,&D); 133 for(int i=1;i<=n;i++){ 134 scanf("%s",str); 135 sscanf(str,"%c-%d",&node[i].flag,&node[i].d); 136 } 137 int low=0,high=D,mid,ans; 138 while(low<=high){ 139 mid=(low+high)>>1; 140 Build(mid); 141 if(SAP(vs,vt)>=2){ 142 ans=mid; 143 high=mid-1; 144 }else 145 low=mid+1; 146 } 147 printf("Case %d: %d\n",t++,ans); 148 } 149 return 0; 150 }
