【hdu3046】【最小割】Pleasant sheep and big big wolf

很直观的最小割模型。

建图方法:1连源点，2连汇点，容量为inf，格子之间连边，容量为1

代码:

#include<cstdio>
#include<cstring>
using namespace std;
const int dx[] = {-1,1,0,0};
const int dy[] = {0,0,-1,1};
const int inf = 0x3f3f3f3f;
const int cns = 200 + 10;
const int maxn = 40000 + 10;
const int maxm = 200000;
struct Edge
{
int c,pos;
int next;
}E[maxm];
int cur[maxn],dis[maxn],pre[maxn],gap[maxn],head[maxn];
int map[cns][cns];
int n,m,kase;
int s,t,nodenum,NE;
void init()
{
freopen("hdu3046.in","r",stdin);
freopen("hdu3046.out","w",stdout);
}

void checkmin(int &a,int b)
{
if(a == -1 || a > b)a = b;
}

void insert(int u,int v,int c)
{
E[NE].c = c;E[NE].pos = v;
E[NE].next = head[u];head[u] = NE++;
E[NE].c = 0;E[NE].pos = u;
E[NE].next = head[v];head[v] = NE++;
}

void build_map()
{
s = 0,t = n * m + 1;
nodenum = t + 1;
for(int i = 1;i <= n;i++)
{
for(int j = 1;j <= m;j++)
{
for(int p = 0;p < 4;p++)
{
int nx = i + dx[p],ny = j + dy[p];
if(nx < 1 || nx > n || ny < 1 || ny > m)continue;
insert((i - 1) * m + j,(nx - 1) * m + ny,1);
}
if(map[i][j] == 1)insert(s,(i - 1) * m + j,inf);
if(map[i][j] == 2)insert((i - 1) * m + j,t,inf);
}
}
}

int sap()
{
memset(dis,0,sizeof(dis));
memset(gap,0,sizeof(gap));
for(int i = s;i <= t;i++)cur[i] = head[i];
int u = pre[s] = s,aug = -1,maxflow = 0;
gap[0] = nodenum;
while(dis[s] < nodenum)
{
loop: for(int &i = cur[u];i != -1;i = E[i].next)
{
int v = E[i].pos;
if(E[i].c && dis[u] == dis[v] + 1)
{
checkmin(aug,E[i].c);
pre[v] = u;
u = v;
if(v == t)
{
maxflow += aug;
for(u = pre[u];v != s;v = u,u = pre[u])
{
E[cur[u]].c -= aug;
E[cur[u]^1].c += aug;
}
aug = -1;
}
goto loop;
}
}
int mind = nodenum;
for(int i = head[u];i != -1;i = E[i].next)
{
int v = E[i].pos;
if(E[i].c && (mind > dis[v]))
{
cur[u] = i;
mind = dis[v];
}
}
if((--gap[dis[u]]) == 0)break;
gap[dis[u] = mind + 1]++;
u = pre[u];
}
return maxflow;
}

void solve()
{
build_map();
printf("Case %d:\n%d\n",++kase,sap());
}

void readdata()
{
kase = 0;
while(~scanf("%d%d",&n,&m))
{
memset(E,0,sizeof(E));
memset(map,0,sizeof(map));
memset(head,-1,sizeof(head));
NE = 0;
for(int i = 1;i <= n;i++)
for(int j = 1;j <= m;j++)
scanf("%d",&map[i][j]);
solve();
}
}

int main()
{
init();
readdata();
return 0;
}