小K的农场

https://www.luogu.org/problemnew/show/P1993

题解：SPFA+差分约束系统

C++版本一

/*
*@Author:   STZG
*@Language: C++
*/
#include <bits/stdc++.h>
#include<iostream>
#include<algorithm>
#include<cstdlib>
#include<cstring>
#include<cstdio>
#include<string>
#include<vector>
#include<bitset>
#include<queue>
#include<deque>
#include<stack>
#include<cmath>
#include<list>
#include<map>
#include<set>
//#define DEBUG
#define RI register int
#define endl "\n"
using namespace std;
typedef long long ll;
//typedef __int128 lll;
const int N=100000+10;
const int M=100000+10;
const int MOD=1e9+7;
const double PI = acos(-1.0);
const double EXP = 1E-8;
const int INF = 0x3f3f3f3f;
int t,n,m,k,p,l,r,u,v;
int ans,cnt,flag,temp,sum;
int a,b,c,dis[N],vis[N];
char str;
struct node{
    int v,w;
    node(){};
    node(int _v,int _w):v(_v),w(_w){}
}x;
vector<node>G[N];
bool SPFA(int u){
    vis[u]=1;
    for(int i=0,j=G[u].size();i<j;i++){
        int v=G[u][i].v;
        if(dis[v]<dis[u]+G[u][i].w){
            dis[v]=dis[u]+G[u][i].w;
            if(vis[v])return 0;
            if(!SPFA(v))return 0;
        }
    }
    vis[u]=0;
    return 1;
}
int main()
{
#ifdef DEBUG
	freopen("input.in", "r", stdin);
	//freopen("output.out", "w", stdout);
#endif
    //ios::sync_with_stdio(false);
    //cin.tie(0);
    //cout.tie(0);
    //scanf("%d",&t);
    //while(t--){
    scanf("%d%d",&n,&m);
    for(int i=1;i<=m;i++){
        scanf("%d",&k);
        if(k==1){
            scanf("%d%d%d",&a,&b,&c);
            G[b].push_back({a,c});
        }else if(k==2){
            scanf("%d%d%d",&a,&b,&c);
            G[a].push_back({b,-c});
        }else if(k==3){
            scanf("%d%d",&a,&b);
            G[a].push_back({b,0});
            G[b].push_back({a,0});
        }
    }
    for(int i=1;i<=n;i++){
        G[0].push_back({i,0});
        dis[i]=-INF;
    }
    cout<<(SPFA(0)?"Yes":"No")<<endl;
    //}

#ifdef DEBUG
	printf("Time cost : %lf s\n",(double)clock()/CLOCKS_PER_SEC);
#endif
    //cout << "Hello world!" << endl;
    return 0;
}

C++版本二

原博客

#include<bits/stdc++.h>
#define il inline
#define For(i,a,b) for(int (i)=(a);(i)<=(b);(i)++)
using namespace std;
const int N=50005,inf=23333333;
int n,m,to[N],net[N],w[N],dis[N],cnt,h[N],tot[N];
bool vis[N];

il int gi(){
    int a=0;char x=getchar();
    while(x<'0'||x>'9')x=getchar();
    while(x>='0'&&x<='9')a=(a<<3)+(a<<1)+x-48,x=getchar();
    return a;
}

il void add(int u,int v,int c){to[++cnt]=v,net[cnt]=h[u],h[u]=cnt,w[cnt]=c;}

il bool spfa(int u){
    vis[u]=1;
    for(int i=h[u];i;i=net[i])
        if(dis[to[i]]<dis[u]+w[i]){
            dis[to[i]]=dis[u]+w[i];
            if(vis[to[i]])return 0;
            if(!spfa(to[i]))return 0;
        }
    vis[u]=0;
    return 1;
}

int main(){
    n=gi(),m=gi();
    int f,a,b,c;
    while(m--){
        f=gi(),a=gi(),b=gi();
        if(f==1)c=gi(),add(b,a,c);
        else if(f==2)c=gi(),add(a,b,-c);
        else if(f==3)add(a,b,0),add(b,a,0);
    }
    For(i,1,n)add(0,i,0),dis[i]=-inf;
    if(!spfa(0))cout<<"No";
    else cout<<"Yes";
    return 0;
}

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