ACdream 1415 Important Roads

  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 }