洛谷P2766 最长不下降子序列问题 网络流_DP

Code:

#include<cstdio>
#include<iostream>
#include<vector>
#include<algorithm>
#include<queue>
#include<cstring>
using namespace std;
const int INF=1000000;
const int maxn=600;
# define  pb push_back
int A[maxn],f[maxn];
int s,t;
int ans=0;
struct Edge{
	int from,to,cap;
	Edge(int u,int v,int c):from(u),to(v),cap(c) {}
};
struct Dicnic{
   vector<Edge>edges;
   vector<int>G[maxn];
   int d[maxn],vis[maxn],cur[maxn];
   queue<int>Q;
   void addedge(int u,int v,int c){
   	edges.pb(Edge(u,v,c));               //正向弧
   	edges.pb(Edge(v,u,0));               //反向弧
   	int m=edges.size();
   	G[u].pb(m-2);
   	G[v].pb(m-1);
   }
   int BFS()
   {
    memset(vis,0,sizeof(vis));
    d[s]=0,vis[s]=1;Q.push(s);
    while(!Q.empty()){
    	int u=Q.front();Q.pop();
    	int sz=G[u].size();
    	for(int i=0;i<sz;++i){
    		Edge e=edges[G[u][i]];
    		if(!vis[e.to]&&e.cap>0){
    			d[e.to]=d[u]+1,vis[e.to]=1;
    			Q.push(e.to);
    		}
    	}
    }
    return vis[t];
   }
   int dfs(int x,int a){
       if(x==t)return a;
       int sz=G[x].size();
       int f,flow=0;
       for(int i=cur[x];i<sz;++i){
       	Edge e=edges[G[x][i]];
        cur[x]=i;
       	if(d[e.to]==d[x]+1&&e.cap>0){
       		f=dfs(e.to,min(a,e.cap));
       		if(f)
       		{
       			int u=G[x][i];
       			a-=f;
                edges[u].cap-=f;
                edges[u^1].cap+=f;
                flow+=f;
                if(a==0)break;
       		}
       	}
       }
       return flow;
   }
   int maxflow(){
   	while(BFS()){
      memset(cur,0,sizeof(cur));
      ans+=dfs(s,INF);
   	}
   	return ans;
   }
}op;
int main()
{
 
    int n;cin>>n;
    for(int i=1;i<=n;++i)cin>>A[i];
    f[1]=1;
    int anss=1;
    for(int i=2;i<=n;++i){
    	f[i]=1;
    	for(int j=1;j<i;++j)
    		if(A[j]<=A[i]){f[i]=max(f[i],f[j]+1);anss=max(anss,f[i]);}
    }
    s=0,t=510;
    for(int i=1;i<=n;++i){
    	if(f[i]==1)op.addedge(s,i,1);
    	if(f[i]==anss)op.addedge(i,t,1);
    }
    for(int i=1;i<n;++i)
    	for(int j=i+1;j<=n;++j)if(f[j]==f[i]+1&&A[j]>=A[i])op.addedge(i,j,1);
    cout<<anss<<endl;
    cout<<op.maxflow()<<endl;
    if(anss==1){cout<<n<<endl;return 0;}
    if(f[1]==anss)op.addedge(1,t,INF);
    if(f[n]==anss)op.addedge(n,t,INF);    //出边
    op.addedge(s,1,INF);
    if(f[n]==1)op.addedge(s,n,INF);        //入边
    for(int i=2;i<=n;++i)
    {
    	if(A[i]>=A[1]&&f[i]==f[1]+1)op.addedge(1,i,INF);
    }
    int fin=op.maxflow();
    if(fin>=INF)
    {
    	int t=fin/INF;
     
    	fin=(fin+t)%INF;
    }
    cout<<fin;
    return 0;
}