ZOJ 1455
2013-10-17 19:41
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不知道如何建图,看了别人报告说要用到差分约束顿时感到很神奇,不过还是没看太明白,建议有算法导论的可以读一下,那个写的更详细和明白,这里用到了行列式不等式求解的知识,可以利用其转化建成图,最后题目就转化为了求最短路问题,但一定要注意负环存在,所以得用Belman-ford算法,以前老用的dijsktra不太管用了,所以又去学习了Bellman算法,果然做题可以推动看书学知识的东西,嘎嘎。
#include<stdio.h> #include<string.h> const int maxn = 10000; const int INF = 0x3f3f3f3f; struct Edge{ int u; int v; int w; }edge[maxn]; int time[maxn]; int d[maxn]; int E,n; void addEdge(int uu,int vv,int ww){ edge[E].u = uu; edge[E].v = vv; edge[E].w = ww; E++; } bool spfa(){ for(int i = 0; i<=n; i++)d[i] = (i==0?0:-INF); d[0]=0; int times=0; while(true){ bool update = false; for(int i=0; i<E; i++){ Edge e=edge[i]; if(d[e.u]!=-INF&&d[e.v]<d[e.u]+e.w){ d[e.v]=d[e.u]+e.w; update = true; } } times++; if(times==n+1)return false; if(!update)return true; } } int main(){ char ch[20]; int kase=1; while(scanf("%d",&n)&&n){ E=0; for(int i=1; i<=n; i++)scanf("%d",&time[i]); while(1){ int u,v; scanf("%s",ch); if(ch[0]=='#')break; scanf("%d %d",&u,&v); if(strcmp(ch,"FAS")==0)addEdge(v,u,-time[u]); else if(strcmp(ch,"FAF")==0)addEdge(v,u,time[v]-time[u]); else if(strcmp(ch,"SAF")==0)addEdge(v,u,time[v]); else if(strcmp(ch,"SAS")==0)addEdge(v,u,0); } for(int i=1;i<=n;i++)addEdge(0,i,0); printf("Case %d:\n",kase); if(spfa())for(int i=1; i<=n; i++)printf("%d %d\n",i,d[i]); else printf("impossible\n"); printf("\n"); kase++; } return 0; }
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