Acdream 1415 important roanalyticdb

Important roadsspecial judgetime limit: 20000/10000 ms (Java/others) memory limit: 128000/64000 KB (Java/others) Problem Description

The city where Georgie lives has n Junctions some of which are connected by bidirectional roads.
Every day Georgie drives from his home to work and back. but the roads in the city where Georgie lives are very bad, so they are very often closed for repair. georgie noticed that when some roads are closed he still can get from home to work in the same time as if all roads were available.

But there are such roads that if they are closed for repair the time Georgie needs to get from home to work increases, and sometimes Georgie even cannot get to work by a car any more. georgie callsuch roads important.
Help Georgie to find all important roads in the city.

Input

The first line of the input file contains N and M-the number of junctions and roads in the city where Georgie lives, respectively (2 ≤ n ≤ 20 000, 1 ≤ m ≤ 100 000 ). georgie lives at the junction 1 and works at the junction n.

The following M lines contain information about roads. each road is specified by the junctions it connects and the time Georgie needs to drive along it. the time to drive along the road is positive and doesn't exceed 100 000. there can be several roads between a pair of junctions, but no road connects a junction to itself. it is guaranteed that if all roads are available, Georgie can get from home to work.

Output output L-the number of important roads-at the first line of the output file. the second line must contain L numbers, the numbers of important roads. roads are numbered from 1 to M as they are given in the input file. sample Input

6 71 2 12 3 12 5 31 3 23 5 12 4 15 6 2

Sample output

25 7

Sourceandrew stankevich contest 22 managermathlover: I think the most interesting thing is to find a cut edge with a heavy edge...

  1 #include <iostream>  2 #include <cstdio>  3 #include <cstring>  4 #include <cmath>  5 #include <algorithm>  6 #include <climits>  7 #include <vector>  8 #include <queue>  9 #include <cstdlib> 10 #include <string> 11 #include <set> 12 #include <stack> 13 #define LL long long 14 #define pli pair<long long,int> 15 #define INF 0x3f3f3f3f3f3fLL 16 using namespace std; 17 const int maxn = 20100; 18 struct arc{ 19     int to,next; 20     LL w; 21     arc(int x = 0,LL y = 0,int z = -1){ 22         to = x; 23         w = y; 24         next = z; 25     } 26 }; 27 struct edge{ 28     int to,id,next; 29     edge(int x = 0,int y = 0,int z = -1){ 30         to = x; 31         id = y; 32         next = z; 33     } 34 }; 35 arc e[maxn*20]; 36 edge g[maxn*20]; 37 int head[maxn],first[maxn],ans[maxn],tot2,num; 38 int n,m,tot,x[maxn*20],y[maxn*20],z[maxn*20]; 39 int dfn[maxn],low[maxn],idx; 40 LL d[2][maxn]; 41 void add(int u,int v,LL w){ 42     e[tot] = arc(v,w,head[u]); 43     head[u] = tot++; 44     e[tot] = arc(u,w,head[v]); 45     head[v] = tot++; 46 } 47 void add2(int u,int v,int id){ 48     g[tot2] = edge(v,id,first[u]); 49     first[u] = tot2++; 50     g[tot2] = edge(u,id,first[v]); 51     first[v] = tot2++; 52 } 53 void dijkstra(int s,LL ds[maxn]){ 54     priority_queue< pli,vector< pli >,greater< pli > >q; 55     bool done[maxn] = {false}; 56     for(int i = 1; i <= n; i++) ds[i] = INF; 57     ds[s] = 0; 58     q.push(make_pair(ds[s],s)); 59     while(!q.empty()){ 60         int u = q.top().second; 61         q.pop(); 62         if(done[u]) continue; 63         done[u] = true; 64         for(int i = head[u]; ~i; i = e[i].next){ 65             if(ds[e[i].to] > ds[u] + e[i].w){ 66                 ds[e[i].to] = ds[u] + e[i].w; 67                 q.push(make_pair(ds[e[i].to],e[i].to)); 68             } 69         } 70     } 71 } 72 void tarjan(int u,int fa){ 73     dfn[u] = low[u] = ++idx; 74     bool flag = true; 75     for(int i = first[u]; ~i; i = g[i].next){ 76         if(g[i].to == fa && flag){ 77             flag = false; 78             continue; 79         } 80         if(!dfn[g[i].to]){ 81             tarjan(g[i].to,u); 82             low[u] = min(low[u],low[g[i].to]); 83             if(low[g[i].to] > dfn[u]) ans[num++] = g[i].id; 84         }else low[u] = min(low[u],dfn[g[i].to]); 85     } 86 } 87 int main(){ 88     while(~scanf("%d %d",&n,&m)){ 89         memset(head,-1,sizeof(head)); 90         memset(first,-1,sizeof(first)); 91         memset(dfn,0,sizeof(dfn)); 92         memset(low,0,sizeof(low)); 93         idx = num = tot2 = tot = 0; 94         for(int i = 1; i <= m; i++){ 95             scanf("%d %d %d",x+i,y+i,z+i); 96             add(x[i],y[i],z[i]); 97         } 98         dijkstra(1,d[0]); 99         dijkstra(n,d[1]);100         LL tmp = d[0][n];101         for(int i = 1; i <= m; i++){102             int u = x[i];103             int v = y[i];104             if(d[0][u] + z[i] + d[1][v] == tmp || d[0][v] + z[i] + d[1][u] == tmp){105                 add2(u,v,i);106             }107         }108         for(int i = 1; i <= n; i++)109             if(!dfn[i]) tarjan(i,-1);110         printf("%d\n",num);111         if(num){112             for(int i = 0; i < num; i++)113                 printf("%d%c",ans[i],i + 1 == num?‘\n‘:‘ ‘);114         }115     }116     return 0;117 }

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Acdream 1415 important roanalyticdb