http://poj.org/problem?id=2415 #include<iostream> #include"queue" #include"algorithm" using namespace std; char edge[51][51]; int sign[51][51][51],n,p1,p2,p3; struct node { int p[3]; int steps; }; queue<node> q; bool init() { int i,j,k; cin>>n>>p1>>p2>>p3; if(n==0)return 0; for(i=0;i<n;i++) for(j=0;j<n;j++) cin>>edge[i][j]; for(i=0;i<n;i++) for(j=0;j<n;j++) for(k=0;k<n;k++) sign[i][j][k]=0; while(!q.empty()) q.pop(); return 1; } inline void set(int a,int b,int c,int v) { sign[a][b][c]=sign[a][c][b]=sign[b][a][c]=sign[b][c][a]=sign[c][a][b]=sign[c][b][a]=v; } int doit() { int i,j; node nd,ndt; nd.p[0]=p1-1; nd.p[1]=p2-1; nd.p[2]=p3-1; nd.steps=1; set(nd.p[0],nd.p[1],nd.p[2],1); q.push(nd); while(!q.empty()) { nd=q.front();q.pop(); if(nd.p[0]==nd.p[1]&&nd.p[1]==nd.p[2]) return sign[nd.p[0]][nd.p[1]][nd.p[2]]; for(i=0;i<n;i++) { for(j=0;j<3;j++) if(!sign[i][nd.p[(j+1)%3]][nd.p[(j+2)%3]]&& edge[nd.p[j]][i]==edge[nd.p[(j+1)%3]][nd.p[(j+2)%3]]) { set(i,nd.p[(j+1)%3],nd.p[(j+2)%3],nd.steps+1); ndt=nd; ndt.steps++; ndt.p[j]=i; q.push(ndt); } } } return 0; } int main() { int ans; while(init()) { ans=doit(); if(!ans)cout<<"impossible"<<endl; else cout<<ans-1<<endl; } return 0; }
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