正题

题目链接:https://www.luogu.org/problemnew/show/P1993

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题目大意

有若干个条件

1. $W_a+w>W_b$
2. $W_a+w<W_b$
3. $W_a=W_b$

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解题思路

差分约束不解释

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$code$

#include<cstdio>
#include<queue>
#include<cstring>
using namespace std;
const int N=11000;
queue<int> q;
struct line{int to,w,next;
}a[N*4];
int n,m,tot,x,y,w,f[N],ls[N],len[N];
bool v[N];
void addl(int x,int y,int w)
{a[++tot].to=y;a[tot].w=w;a[tot].next=ls[x];ls[x]=tot;
}
bool spfa(int x){v[x]=1;for(int i=ls[x];i;i=a[i].next){int y=a[i].to;if(f[y]>=f[x]+a[i].w) continue;f[y]=f[x]+a[i].w;if(v[y]) return 0;if(!spfa(y)) return 0; }v[x]=0;return 1;
}
int main()
{scanf("%d%d",&n,&m);for(int i=1;i<=m;i++){int S,x,y,w;scanf("%d%d%d",&S,&x,&y);if(S==1){scanf("%d",&w);addl(y,x,w);}if(S==2){scanf("%d",&w);addl(x,y,-w);}if(S==3){addl(x,y,0);addl(y,x,0);}}for(int i=1;i<=n;i++)addl(0,i,0),f[i]=-6666666;if(spfa(0)) printf("Yes");else printf("No");
}