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 }

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