[倍增Floyd] BZOJ 1706 [usaco2007 Nov]relays 奶牛接力跑

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题目大意：求s到t 长度为n 的最短路

裸的倍增Floyd

#include<cstdio>#include<cstdlib>#include<algorithm>#include<cstring>using namespace std;inline char nc(){static char buf[100000],*p1=buf,*p2=buf;if (p1==p2) { p2=(p1=buf)+fread(buf,1,100000,stdin); if (p1==p2) return EOF; }return *p1++;}inline void read(int &x){char c=nc(),b=1;for (;!(c>='0' && c<='9');c=nc()) if (c=='-') b=-1;for (x=0;c>='0' && c<='9';x=x*10+c-'0',c=nc()); x*=b;}const int N=205;struct edge{int u,v,w;}G[N];int n,m,S,T;int f[N][N],h[N][N],tmp[N][N];int sx[N],icnt;inline int Bin(int x){return lower_bound(sx+1,sx+icnt+1,x)-sx;}int main(){freopen("t.in","r",stdin);freopen("t.out","w",stdout);read(n); read(m); read(S); read(T);for (int i=1;i<=m;i++)read(G[i].w),read(G[i].u),read(G[i].v),sx[++icnt]=G[i].u,sx[++icnt]=G[i].v;sort(sx+1,sx+icnt+1);icnt=unique(sx+1,sx+icnt+1)-sx-1;for (int i=1;i<=m;i++)G[i].u=Bin(G[i].u),G[i].v=Bin(G[i].v);memset(f,0x3f,sizeof(f));memset(h,0x3f,sizeof(h));for (int i=1;i<=icnt;i++)h[i][i]=0;for (int i=1;i<=m;i++)f[G[i].u][G[i].v]=f[G[i].v][G[i].u]=G[i].w;for (int t=0;(1<<t)<=n;t++){if (n&(1<<t)){memset(tmp,0x3f,sizeof(tmp));for (int k=1;k<=icnt;k++)for (int i=1;i<=icnt;i++)for (int j=1;j<=icnt;j++)tmp[i][j]=min(tmp[i][j],h[i][k]+f[k][j]);memcpy(h,tmp,sizeof(tmp));}memset(tmp,0x3f,sizeof(tmp));for (int k=1;k<=icnt;k++)for (int i=1;i<=icnt;i++)for (int j=1;j<=icnt;j++)tmp[i][j]=min(tmp[i][j],f[i][k]+f[k][j]);memcpy(f,tmp,sizeof(tmp));}printf("%d\n",h[Bin(S)][Bin(T)]);return 0;}

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