【Luogu】P2219修筑绿化带（单调队列）

#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<algorithm>
#include<cctype>
#include<queue>
#include<vector>
#define maxm 2000020
#define maxn 1010000
using namespace std;
inline long long read(){
    long long num=0,f=1;
    char ch=getchar();
    while(!isdigit(ch)){
        if(ch==‘-‘)    f=-1;
        ch=getchar();
    }
    while(isdigit(ch)){
        num=num*10+ch-‘0‘;
        ch=getchar();
    }
    return num*f;
}

inline int count(int i){    return i&1?i+1:i-1;    }

struct Edge{
    int next,to,val;
}edge[maxm*2];
int head[maxn],num;
inline void addedge(int from,int to,int val){
    edge[++num]=(Edge){head[from],to,val};
    head[from]=num;
}
inline void add(int from,int to,int val){
    addedge(from,to,val);
    addedge(to,from,0);
}

int dfn[maxn];
bool vis[maxn];
int Start,End;
int last[maxm],tot;

bool bfs(){
    for(register int i=1;i<=tot;++i)    vis[last[i]]=0;    tot=0;
    dfn[Start]=1;    vis[Start]=1;
    queue<int>q;    q.push(Start);
    while(!q.empty()){
        int from=q.front();    q.pop();
        last[++tot]=from;
        for(int i=head[from];i;i=edge[i].next){
            int to=edge[i].to;
            if(vis[to]||edge[i].val==0)    continue;
            dfn[to]=dfn[from]+1;
            vis[to]=1;    q.push(to);
        }
    }
    return vis[End];
}

int dfs(int x,int val,int limit){
    if(x<=10000&&x>limit)    return 0;
    if(val==0||x==End)    return val;
    int flow=0;    vis[x]=1;
    last[++tot]=x;
    for(int i=head[x];i;i=edge[i].next){
        int to=edge[i].to;
        if(edge[i].val==0||vis[to]||dfn[to]!=dfn[x]+1)    continue;
        int now=dfs(to,min(val,edge[i].val),limit);
        edge[i].val-=now;    edge[count(i)].val+=now;    val-=now;    flow+=now;
        if(val<=0)    break;
    }
    if(flow!=val)    dfn[x]=-1;
    return flow;
}

int maxflow(int limit){
    int ans=0;
    while(bfs()){
        for(register int i=1;i<=tot;++i)    vis[last[i]]=0;    tot=0;
        int now=dfs(Start,0x7fffffff,limit);
        if(!now)    break;
        return now;
    }
    return ans;
}

int main(){
    int n=read();
    End=n+10000+1;
    for(int i=1;i<=n;++i){
        int a=read(),b=read();
        add(a,i+10000,1);
        add(b,i+10000,1);
        add(i+10000,End,1);
    }
    int ans;
    for(ans=1;;++ans){
        add(Start,ans,1);
        if(maxflow(ans)==0)    break;
    }
    printf("%d",ans-1);
    return 0;
}