luogu P1608 路径统计

analysis

最短路计数

注意当spfa中一个点计算完了后，一定要记得将它清零，否则会造成重复计算，如下图：

同时，记得处理重边，否则会导致路径数偏多（题目认为城市u，v之间所有等长的边都是同一条路）

code

// luogu-judger-enable-o2
#include
using namespace std;
#define loop(i,start,end) for(int i=start;i<=end;++i)
#define anti_loop(i,start,end) for(int i=start;i>=end;--i)
#define clean(arry,num) memset(arry,num,sizeof(arry))
#define ll int
#define isdegit(a) ((a>='0'&&a<='9'))
template<typename T>void read(T &x){
    x=0;char r=getchar();T neg=1;
    while(!isdegit(r)){if(r=='-')neg=-1;r=getchar();}
    while(isdegit(r)){x=(x<<1)+(x<<3)+r-'0';r=getchar();}
    x*=neg;
}
const int maxn=2000+10,maxm=maxn*maxn;
struct node{
    int e;
    int w;
    int nxt;
}edge[maxm];
int n,m,cnt=0;
int head[maxn];

inline void addl(register int u,register int v,register int w){
    edge[cnt].e=v;
    edge[cnt].w=w;
    edge[cnt].nxt=head[u];
    head[u]=cnt++;
}

ll dis[maxn];
ll rot[maxn];
deque<int>q;
int Dis[maxn][maxn];

inline void spfa(register int u,register int to){
    clean(dis,0x7f);
    clean(rot,0);
    dis[u]=0;
    rot[u]=1; 
    q.push_front(u);
    while(q.empty()==false){
        int f=q.front();
        q.pop_front();
        
        if(f==to)
            continue;
        for(int i=head[f];i!=-1;i=edge[i].nxt){
            int v=edge[i].e;
            if(dis[v]>dis[f]+(ll)Dis[f][v]){
                rot[v]=rot[f];
                dis[v]=dis[f]+(ll)Dis[f][v];
                if(!q.empty()&&dis[q.front()]>dis[v])
                    q.push_front(v);
                else q.push_back(v);
            }
            else if(dis[v]==dis[f]+(ll)Dis[f][v]){
                rot[v]+=rot[f];
                q.push_back(v);
            }
        }
        rot[f]=0;
    }
    if(dis[to]<0x7f7f7f7f)
        printf("%d %d\n",dis[to],rot[to]);
    else
        printf("No answer\n");
}

int main(){
    #ifndef ONLINE_JUDGE
    freopen("dat.txt","r",stdin);
    #endif
    
    clean(head,-1);
    clean(Dis,0x7f);
    read(n);
    read(m);
    register int x,y,z;
    loop(i,1,m){
        read(x);read(y);read(z);
        Dis[x][y]=min(Dis[x][y],z);
    }
    loop(i,1,n){
        loop(j,1,n){
            if(Dis[i][j]<0x7f7f7f7f){
                addl(i,j,Dis[i][j]);
            }
        }
    }
    
    spfa(1,n);
    
    return 0;
}