uva 11380(最大流+拆点）

题目链接：http://acm.hust.edu.cn/vjudge/problem/viewProblem.action?id=36707

思路：根据题意拆点建图即可。

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<queue>
using namespace std;
#define MAXN 2222
#define inf 1<<30

struct Edge{
    int v,cap,next;
}edge[MAXN*MAXN];

int n,m,p,NE,NV,vs,vt;
int head[MAXN];

void Insert(int u,int v,int cap)
{
    edge[NE].v=v;
    edge[NE].cap=cap;
    edge[NE].next=head[u];
    head[u]=NE++;

    edge[NE].v=u;
    edge[NE].cap=0;
    edge[NE].next=head[v];
    head[v]=NE++;
}

int level[MAXN],gap[MAXN];
void bfs(int vt)
{
    memset(level,-1,sizeof(level));
    memset(gap,0,sizeof(gap));
    level[vt]=0;
    gap[0]++;
    queue<int>que;
    que.push(vt);
    while(!que.empty()){
        int u=que.front();
        que.pop();
        for(int i=head[u];i!=-1;i=edge[i].next){
            int v=edge[i].v;
            if(level[v]!=-1)continue;
            level[v]=level[u]+1;
            gap[level[v]]++;
            que.push(v);
        }
    }
}

int cur[MAXN],pre[MAXN];
int SAP(int vs,int vt)
{
    bfs(vt);
    memset(pre,-1,sizeof(pre));
    memcpy(cur,head,sizeof(head));
    int u=pre[vs]=vs,aug=inf,maxflow=0;
    gap[0]=NV;
    while(level[vs]<NV){
        bool flag=false;
        for(int &i=cur[u];i!=-1;i=edge[i].next){
            int v=edge[i].v;
            if(edge[i].cap>0&&level[u]==level[v]+1){
                flag=true;
                aug=min(aug,edge[i].cap);
                pre[v]=u;
                u=v;
                if(v==vt){
                    maxflow+=aug;
                    for(u=pre[u];v!=vs;v=u,u=pre[u]){
                        edge[cur[u]].cap-=aug;
                        edge[cur[u]^1].cap+=aug;
                    }
                    aug=inf;
                }
                break;
            }
        }
        if(flag)continue;
        int minlevel=NV;
        for(int i=head[u];i!=-1;i=edge[i].next){
            int v=edge[i].v;
            if(edge[i].cap>0&&level[v]<minlevel){
                minlevel=level[v];
                cur[u]=i;
            }
        }
        if(--gap[level[u]]==0)break;
        level[u]=minlevel+1;
        gap[level[u]]++;
        u=pre[u];
    }
    return maxflow;
}

char map[44][44];
int dir[4][2]={{-1,0},{1,0},{0,-1},{0,1}};

int main()
{
    while(~scanf("%d%d%d",&n,&m,&p)){
        for(int i=0;i<n;i++)scanf("%s",map[i]);
        NE=0;
        vs=0,vt=2*n*m+1,NV=2*n*m+2;
        memset(head,-1,sizeof(head));
        for(int i=0;i<n;i++){
            for(int j=0;j<m;j++){
                if(map[i][j]=='~')continue;
                for(int k=0;k<4;k++){
                    int x=i+dir[k][0],y=j+dir[k][1];
                    if(x<0||x>=n||y<0||y>=m)continue;
                    Insert(i*m+j+1+n*m,x*m+y+1,inf);
                }
                if(map[i][j]=='*'){
                    Insert(vs,i*m+j+1,1);
                    Insert(i*m+j+1,i*m+j+1+n*m,1);
                }
                else if(map[i][j]=='.')Insert(i*m+j+1,i*m+j+1+n*m,1);
                else if(map[i][j]=='@')Insert(i*m+j+1,i*m+j+1+n*m,inf);
                else if(map[i][j]=='#')Insert(i*m+j+1,i*m+j+1+n*m,inf),Insert(i*m+j+1+n*m,vt,p);
            }
        }
        printf("%d
",SAP(vs,vt));
    }
    return 0;
}