answer:
Farm a has at least c more crops than farm b;
this sentence shows: a-b> = c
can be transformed into b-a <=-c
add(a,b,-c)
farm a is more than farm b At most c units of crops are planted;
explanation: a-b <= c
add(b,a,c)
farm a and farm b have the same number of crops.
Description a-b <= 0
b-a <= 0
add(a,b,0)
add(b,a,0)
Code:
#include<bits/stdc++.h>
#define MAXN 50005
#define inf int(16843009)
using namespace std;
int N,M,s,t;
struct edge{
int v,w;
edge(int v=0, int w=0):v(v),w(w){
};
};
vector<edge> adj[MAXN];
int d[MAXN], cnt[MAXN];
bool inq[MAXN];
bool spfa(int s){
memset(d, 1, sizeof(d));
queue<int> q;
d[s] = 0; q.push(s); inq[s] = 1;
int u,v,w;
while(!q.empty()){
u = q.front(); q.pop(); inq[u] = 0;
for(int i=0;i<adj[u].size();i++){
v = adj[u][i].v; w = adj[u][i].w;
if(d[u] + w < d[v]){
d[v] = d[u] + w;
cnt[v] = cnt[u] + 1;//cnt[v] = s->v最短路包含的边数
if(cnt[v] >= N+1) return 1;//判负环
if(inq[v]==0){
q.push(v); inq[v] = 1;
}
}
}
}
return 0;
}
int main(){
//freopen("P1993_2.in","r",stdin);
scanf("%d%d", &N, &M);
int d,u,v,w;
for(int i=1;i<=M;i++){
cin>>d;
if(d==1)
{
scanf("%d%d%d", &v, &u, &w);
adj[v].push_back(edge(u,-w));
}
else if(d==2)
{
scanf("%d%d%d", &v, &u, &w);
adj[u].push_back(edge(v,w));
}
else if(d==3)
{
scanf("%d%d", &v, &u);
adj[u].push_back(edge(v,0));
adj[v].push_back(edge(u,0));
}
}
s = 0;
for(int i=1;i<=N;i++){
adj[0].push_back(edge(i,0));
}
if(spfa(0)){
printf("No\n");
}
else
cout<<"Yes"<<endl;
return 0;
}