题面
https://www.luogu.org/problemnew/show/P2766
我也不知道为什么全合肥八中(hfyz2018ji37)就我一个蒟蒻写了费用流。
#include<cstdio> #include<cstring> #include<iostream> #include<vector> #include<queue> #define N 1500 #define INF 1000000007 #define T (2*n+1) #define S 0 #define LL long long #define ri register int using namespace std; int n,a[N]; LL ans1,ans2; struct graph { vector<int> to,w,c; vector<int> ed[N]; LL dis[N]; int cur[N]; bool vis[N]; void clear() { for (ri i=S;i<=T;i++) ed[i].clear(); to.clear(); w.clear(); c.clear(); } void add_edge(int a,int b,int aw,int ac) { to.push_back(b); w.push_back(aw); c.push_back(ac); ed[a].push_back(to.size()-1); to.push_back(a); w.push_back(0); c.push_back(-ac); ed[b].push_back(to.size()-1); } bool spfa() { memset(dis,-0x3f,sizeof(dis)); memset(vis,0,sizeof(vis)); queue<int> q; dis[S]=0;q.push(S);vis[S]=1; while (!q.empty()) { int x=q.front(); q.pop(); for (ri i=0;i<ed[x].size();i++) { int e=ed[x][i]; if (dis[to[e]]<dis[x]+c[e] && w[e]) { dis[to[e]]=dis[x]+c[e]; if (!vis[to[e]]) vis[to[e]]=1,q.push(to[e]); } } vis[x]=0; } return dis[T]>-INF; } int dfs(int x,int lim) { if (x==T || !lim) return lim; LL sum=0; vis[x]=1; for (ri &i=cur[x];i<ed[x].size();i++) { int e=ed[x][i]; if (dis[x]+c[e]==dis[to[e]] && w[e] && !vis[to[e]]) { int f=dfs(to[e],min(lim,w[e])); w[e]-=f; w[1^e]+=f; lim-=f; sum+=f; if (!lim) return sum; } } return sum; } void work() { spfa(); ans1=dis[T]; memset(vis,0,sizeof(vis)); memset(cur,0,sizeof(cur)); ans2=dfs(S,INF); while (spfa()) { memset(vis,0,sizeof(vis)); memset(cur,0,sizeof(cur)); int t=dfs(S,INF); if (dis[T]==ans1) ans2+=t; } } LL zkw(int s) { LL ret=0; while (spfa()) { memset(vis,0,sizeof(vis)); memset(cur,0,sizeof(cur)); int t=dfs(S,INF); if (dis[T]==s) ret+=t; } return ret; } } G; int main() { scanf("%d",&n); for (ri i=1;i<=n;i++) scanf("%d",&a[i]); for (ri i=1;i<=n;i++) { G.add_edge(S,i,INF,0); G.add_edge(i,n+i,1,1); G.add_edge(n+i,T,INF,0); for (ri j=i+1;j<=n;j++) if (a[i]<=a[j]) G.add_edge(n+i,j,INF,0); } G.work(); cout<<ans1<<endl<<ans2<<endl; G.clear(); for (ri i=1;i<=n;i++) { G.add_edge(S,i,INF,0); if (i==1 || i==n) G.add_edge(i,n+i,INF,1); else G.add_edge(i,n+i,1,1); G.add_edge(n+i,T,INF,0); for (ri j=i+1;j<=n;j++) if (a[i]<=a[j]) G.add_edge(n+i,j,1,0); } if (ans1==1) cout<<ans2<<endl; else cout<<G.zkw(ans1)<<endl; }