Marriage Match IV
Time Limit: 2000/1000 MS (Java/Others) Memory Limit: 32768/32768 K (Java/Others)
Total Submission(s): 1514 Accepted Submission(s): 427
Problem Description Do not sincere non-interference。
Like that show, now starvae also take part in a show, but it take place between city A and B. Starvae is in city A and girls are in city B. Every time starvae can get to city B and make a data with a girl he likes. But there are two problems with it, one is starvae must get to B within least time, it’s said that he must take a shortest path. Other is no road can be taken more than once. While the city starvae passed away can been taken more than once.
So, under a good RP, starvae may have many chances to get to city B. But he don’t know how many chances at most he can make a data with the girl he likes . Could you help starvae?
Input The first line is an integer T indicating the case number.(1<=T<=65)
For each case,there are two integer n and m in the first line ( 2<=n<=1000, 0<=m<=100000 ) ,n is the number of the city and m is the number of the roads.
Then follows m line ,each line have three integers a,b,c,(1<=a,b<=n,0<c<=1000)it means there is a road from a to b and it’s distance is c, while there may have no road from b to a. There may have a road from a to a,but you can ignore it. If there are two roads from a to b, they are different.
At last is a line with two integer A and B(1<=A,B<=N,A!=B), means the number of city A and city B.
There may be some blank line between each case.
Output Output a line with a integer, means the chances starvae can get at most.
Sample Input 3 7 8 1 2 1 1 3 1 2 4 1 3 4 1 4 5 1 4 6 1 5 7 1 6 7 1 1 7 6 7 1 2 1 2 3 1 1 3 3 3 4 1 3 5 1 4 6 1 5 6 1 1 6 2 2 1 2 1 1 2 2 1 2
Sample Output 2 1 1
Author starvae@HDU
Source
HDOJ Monthly Contest – 2010.06.05
这题就是求从A到B的最短路径的条数。
一条边只能经过一次。
先通过最短路去除掉没有用的边。
然后用一次最大流就是答案了。
从A和B分别出发求最短路dist1,dist2.
注意从B求得额时候要反向。
如果dist1[a]+dist2[b]+c==dist1[B].那么这条边就是有用的。。
我用的SPFA求最短路的。
//============================================================================ // Name : HDU.cpp // Author : // Version : // Copyright : Your copyright notice // Description : Hello World in C++, Ansi-style //============================================================================ #include <iostream> #include <stdio.h> #include <string.h> #include <algorithm> using namespace std; const int MAXN=2010; const int MAXM=2000010; const int INF=0x3f3f3f3f; //最大流SAP struct Node { int to,next,cap; }edge[MAXM]; int tol; int head[MAXN]; int gap[MAXN],dis[MAXN],pre[MAXN],cur[MAXN]; void init() { tol=0; memset(head,-1,sizeof(head)); } void addedge(int u,int v,int w,int rw=0) { edge[tol].to=v;edge[tol].cap=w;edge[tol].next=head[u];head[u]=tol++; edge[tol].to=u;edge[tol].cap=rw;edge[tol].next=head[v];head[v]=tol++; } int sap(int start,int end,int nodenum) { memset(dis,0,sizeof(dis)); memset(gap,0,sizeof(gap)); memcpy(cur,head,sizeof(head)); int u=pre[start]=start,maxflow=0,aug=-1; gap[0]=nodenum; while(dis[start]<nodenum) { loop: for(int &i=cur[u];i!=-1;i=edge[i].next) { int v=edge[i].to; if(edge[i].cap&&dis[u]==dis[v]+1) { if(aug==-1||aug>edge[i].cap) aug=edge[i].cap; pre[v]=u; u=v; if(v==end) { maxflow+=aug; for(u=pre[u];v!=start;v=u,u=pre[u]) { edge[cur[u]].cap-=aug; edge[cur[u]^1].cap+=aug; } aug=-1; } goto loop; } } int mindis=nodenum; for(int i=head[u];i!=-1;i=edge[i].next) { int v=edge[i].to; if(edge[i].cap&&mindis>dis[v]) { cur[u]=i; mindis=dis[v]; } } if((--gap[dis[u]])==0)break; gap[dis[u]=mindis+1]++; u=pre[u]; } return maxflow; } //SPFA int first[MAXN]; bool vis[MAXN]; int cnt[MAXN]; int que[MAXN]; int dist[MAXN]; struct Edge { int to,v,next; }edge1[MAXM]; int tt; void add(int a,int b,int v) { edge1[tt].to=b; edge1[tt].v=v; edge1[tt].next=first[a]; first[a]=tt++; } bool SPFA(int start,int n) { int front=0,rear=0; for(int v=1;v<=n;v++) { if(v==start) { que[rear++]=v; vis[v]=true; cnt[v]=1; dist[v]=0; } else { vis[v]=false; cnt[v]=0; dist[v]=INF; } } while(front!=rear) { int u=que[front++]; vis[u]=false; if(front>=MAXN)front=0; for(int i=first[u];i!=-1;i=edge1[i].next) { int v=edge1[i].to; if(dist[v]>dist[u]+edge1[i].v) { dist[v]=dist[u]+edge1[i].v; if(!vis[v]) { vis[v]=true; que[rear++]=v; if(rear>=MAXN)rear=0; if(++cnt[v]>n)return false; } } } } return true; } int a[100010],b[100010],c[100010]; int dist1[MAXN],dist2[MAXN]; int main() { //freopen("in.txt","r",stdin); //freopen("out.txt","w",stdout); int T; int n,m; scanf("%d",&T); while(T--) { scanf("%d%d",&n,&m); int A,B; for(int i=0;i<m;i++) scanf("%d%d%d",&a[i],&b[i],&c[i]); scanf("%d%d",&A,&B); tt=0; memset(first,-1,sizeof(first)); for(int i=0;i<m;i++) add(a[i],b[i],c[i]); SPFA(A,n); // if(dist[B]==INF) // { // printf("0\n"); // continue; // } memcpy(dist1,dist,sizeof(dist)); tt=0; memset(first,-1,sizeof(first)); for(int i=0;i<m;i++) add(b[i],a[i],c[i]); SPFA(B,n); memcpy(dist2,dist,sizeof(dist)); init(); for(int i=0;i<m;i++) { if(a[i]!=b[i] && dist1[a[i]]+dist2[b[i]]+c[i]==dist1[B]) addedge(a[i],b[i],1); } printf("%d\n",sap(A,B,n)); } return 0; }
