3681. 中位数

Aimeeeee

把原图改造一下\(>=mid\)那么权值为1，反之为0

然后跑最长路，如果最长路不小于0，那么就可行，反之不行

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
#include<queue>
#define int long long
using namespace std;
int n,m;
long long Aimee[1000001];
int x,y;
queue<int> q;
struct b{
   int to;
   int ne;
} ed[1000001];
int Ai;
int head[1000001];
int vis[1000001];
int dp[1000001];
int tem[1000001];
void add(int f,int to){
   Ai++;
   ed[Ai].to=to;
   ed[Ai].ne=head[f];
   head[f]=Ai;
   return ;
}
long long l,r;
int dfs(int no,int k){
   if(vis[no])
   return dp[no];
   vis[no]=1;
   for(int i=head[no];i;i=ed[i].ne){
   	dp[no]=max(dp[no],dfs(ed[i].to,k)+tem[ed[i].to]);
   }
   return dp[no];
}
bool check(int now){
   for(int i=0;i<=n;++i){
   	tem[i]=(Aimee[i]>=now? 1 :-1);
   	vis[i]=0;
   	dp[i]=-999999999;
   }
   	dp[n]=0;
   	vis[n]=1;
   dfs(1,now);
   dp[1]+=tem[1];
   return dp[1]>=0;
}
signed main(){
   scanf("%d%d",&n,&m);
   for(int i=1;i<=n;++i){
   	scanf("%lld",&Aimee[i]);
   }
   for(int i=1;i<=m;++i){
   	scanf("%d%d",&x,&y);
   	add(x,y); 
   }
   int ans;
   ans=r=9999999990;
   
   while(l<=r){
   	long long mid=l+(r-l)/2;
   	if(check(mid)){
   		l=mid+1;
   		ans=mid;
   	}else{
   		r=mid-1;
   	}
   }
   cout<<(ans==(long long)9999999990?-1 :ans);
   return 0;
}