[BZOJ1779][Usaco2010 Hol]Cowwar 奶牛战争（最大流）

题目描述

传送门

题解

一眼网络流啊。。。
对于所有的J，s->i,1
对于所有的T，i->t,1
将J和E再拆两个点xi,yi，连边xi->yi,1
对于每一个J能移动到的位置（J或E），连边i->xj,1
对于每一个位置（J或E）能攻击到的T，连边yi->j,1
跑最大流即可
即用流来模拟了攻击的过程

代码

#include
#include
#include
#include
#include
#include
using namespace std;
#define N 160000
#define inf 2100000000

int n,m,s,t,maxflow;bool e[1005][1005];
char str[N];
int tot,point[N],nxt[N],v[N],remain[N];
int deep[N],last[N],cur[N],num[N];
queue <int> q;

void addedge(int x,int y,int cap)
{
    ++tot; nxt[tot]=point[x]; point[x]=tot; v[tot]=y; remain[tot]=cap;
    ++tot; nxt[tot]=point[y]; point[y]=tot; v[tot]=x; remain[tot]=0;
}
void bfs(int t)
{
    for (int i=1;i<=t;++i) deep[i]=t;
    deep[t]=0;
    for (int i=1;i<=t;++i) cur[i]=point[i];
    q.push(t);
    while (!q.empty())
    {
        int now=q.front();q.pop();
        for (int i=point[now];i!=-1;i=nxt[i])
            if (deep[v[i]]==t&&remain[i^1])
            {
                deep[v[i]]=deep[now]+1;
                q.push(v[i]);
            }
    }
}
int addflow(int s,int t)
{
    int now=t,ans=inf;
    while (now!=s)
    {
        ans=min(ans,remain[last[now]]);
        now=v[last[now]^1];
    }
    now=t;
    while (now!=s)
    {
        remain[last[now]]-=ans;
        remain[last[now]^1]+=ans;
        now=v[last[now]^1];
    }
    return ans;
}
void isap(int s,int t)
{
    bfs(t);
    for (int i=1;i<=t;++i) ++num[deep[i]];

    int now=s;
    while (deep[s]if (now==t)
        {
            maxflow+=addflow(s,t);
            now=s;
        }

        bool has_find=0;
        for (int i=cur[now];i!=-1;i=nxt[i])
            if (deep[v[i]]+1==deep[now]&&remain[i])
            {
                has_find=1;
                cur[now]=i;
                last[v[i]]=i;
                now=v[i];
                break;
            }

        if (!has_find)
        {
            int minn=t-1;
            for (int i=point[now];i!=-1;i=nxt[i])
                if (remain[i]) minn=min(minn,deep[v[i]]);
            if (!(--num[deep[now]])) break;
            ++num[deep[now]=minn+1];
            cur[now]=point[now];
            if (now!=s) now=v[last[now]^1];
        }
    }
}
int main()
{
    scanf("%d%d",&n,&m);
    scanf("%s",str+1);
    for (int i=1;i<=m;++i)
    {
        int x,y;scanf("%d%d",&x,&y);
        e[x][y]=e[y][x]=1;
    }
    s=3*n+1,t=s+1;
    tot=-1;memset(point,-1,sizeof(point));
    for (int i=1;i<=n;++i)
    {
        if (str[i]=='J')
        {
            addedge(s,i,1);
            addedge(i,n+i,1);
            addedge(n+i,n+n+i,1);
            for (int j=1;j<=n;++j)
                if (e[i][j]&&str[j]!='T') addedge(i,n+j,1);
        }
        if (str[i]=='T')
        {
            addedge(i,t,1);
            for (int j=1;j<=n;++j)
                if (e[i][j]&&str[j]!='T') addedge(n+n+j,i,1);
        }
        if (str[i]=='E') addedge(n+i,n+n+i,1);
    }
    isap(s,t);
    printf("%d\n",maxflow);
}