[题解]luogu_P3084（差分约束/梦想spfa

https://www.luogu.org/blog/FakeSilhouette/solution-p3084

虽然是道dp但是学到了暴力spfa+1sspfa黑框spfa膜蛤spfa梦想spfa

双端队列spfa，那个质数判负环

#include<iostream>
#include<cstdio>
#include<cstring>
#include<queue>
#define pb push_back
#define pf push_front
#define ppf pop_front
#define ppb pop_back
using namespace std;
const int maxn=1000009;
const int inf=1e9+7;
struct node{
    int v,w,nxt;    
}e[maxn*3];
int head[maxn],d[maxn],vis[maxn],cnt,n,mn,mx,m;
void add(int u,int v,int w){
    e[++cnt].v=v;
    e[cnt].nxt=head[u];
    e[cnt].w=w;
    head[u]=cnt;
}
int cntt[maxn];int tot=0;
int spfa(){
    memset(d,0x3f,sizeof(d));
    deque<int>q;
    q.push_back(0);d[0]=0;vis[0]=1;
    while(!q.empty()){
        int x=q.front();q.pop_front();vis[x]=0;
        for(int i=head[x];i;i=e[i].nxt){
            int y=e[i].v;
            if(d[y]>d[x]+e[i].w){
                d[y]=d[x]+e[i].w;
//                cntt[y]=cntt[x]+1; 
                if(!vis[y]){
                    vis[y]=1;
//                    if(cntt[y]>n)return -1;
                    if(++tot>1926817*5)return -1;//大点约785ms 
                    if(!q.empty()&&d[y]>d[q.front()])q.push_back(y);
                    else q.push_front(y);
                }
            }
        }
    }
    return d[n];
}
int main(){
    scanf("%d%d",&n,&m);
    for(int i=0,u,v;i<m;i++){
        scanf("%d%d",&u,&v);
        add(u-1,v,1);
        add(v,u-1,-1);
    }
    for(int i=1;i<=n;i++){
        add(i-1,i,1);add(i,i-1,0);
    }
    printf("%d",spfa());
}