关于差分约束的介绍可以直接看wikioi
链接在这里：差分约束 - OI Wiki (oi-wiki.org)
算法学习笔记(11): 差分约束 - 知乎 (zhihu.com)
1 #include "bits/stdc++.h"
2 using namespace std;
3 typedef long long LL;
4 const int MAX=1e5+5;
5 int n,m,ans;
6 int tot,head[MAX],adj[MAX<<1],nxt[MAX<<1],wei[MAX<<1],dis[MAX],cnt[MAX];
7 queue <int> q;
8 bool t[MAX];
9 struct Edge{int u,v,w1,w2;bool operator <(const Edge &tt) const {return w1<tt.w1;}}edge[MAX];
10 inline int read(){
11 int an=0,x=1;char c=getchar();
12 while (c<'0' || c>'9') {if (c=='-') x=-1;c=getchar();}
13 while (c>='0' && c<='9') {an=an*10+c-'0';c=getchar();}
14 return an*x;
15 }
16 void addedge(int u,int v,int w){
17 tot++;
18 adj[tot]=v;wei[tot]=w;
19 nxt[tot]=head[u];
20 head[u]=tot;
21 }
22 bool spfa(){
23 int i,j,u,v;
24 for (i=1;i<=n;i++) cnt[i]=0;
25 while (!q.empty()){
26 u=q.front();q.pop();t[u]=false;
27 if (cnt[u]>n) return false;
28 for (i=head[u];i;i=nxt[i]){
29 v=adj[i];
30 if (dis[v]>dis[u]+wei[i]){
31 dis[v]=dis[u]+wei[i];
32 if (!t[v]){
33 q.push(v),t[v]=true;
34 cnt[v]++;
35 }
36 }
37 }
38 }
39 return true;
40 }
41 int main(){
42 freopen ("f.in","r",stdin);
43 freopen ("f.out","w",stdout);
44 int i,j;
45 n=read();m=read();
46 for (i=1;i<=m;i++){
47 edge[i].u=read();
48 edge[i].v=read();
49 edge[i].w1=read();
50 addedge(edge[i].v,edge[i].u,edge[i].w1);
51 }
52 for (i=1;i<=n;i++) addedge(n+1,i,0);
53 memset(t,false,sizeof(t));
54 for (i=1;i<=n+1;i++) dis[i]=1e9;
55 q.push(n+1);t[n+1]=true;dis[n+1]=0;
56 if (spfa()) printf("YES");
57 else printf("NO");
58 return 0;
59
