102 lines
1.9 KiB
C++
102 lines
1.9 KiB
C++
#include <cstdio>
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#include <cstdlib>
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#include <cstring>
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#include <vector>
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#include <queue>
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#include <list>
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#include <set>
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#include <map>
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#include <algorithm>
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using namespace std;
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#define INF 1e9+7
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#define MAXN 5000
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#define MAXR 100000
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int N,R;
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struct edge
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{
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edge(int To,int Dist):to(To),dist(Dist){}
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int to;
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int dist;
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};
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vector<edge> G[MAXN+1];
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/// 最短路: dis[i]=从1号路口出发到i路口的最短距离
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int dis[MAXN+1];
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/// 次短路: second_dis[i]=从1号路口出发到i路口的次短距离
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int second_dis[MAXN+1];
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/// PR.first 为距离, PR.second 为点
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typedef pair<int,int> PR;
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void solve()
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{
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priority_queue< PR,vector<PR>,greater<PR> > bus;
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fill(dis,dis+N,INF);
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fill(second_dis,second_dis+N,INF);
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dis[0]=0;
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bus.push(PR(0,0));
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while(!bus.empty())
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{
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PR p=bus.top();
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bus.pop();
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int d=p.first;
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int v=p.second;
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if(second_dis[v]<d)
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{
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/// 如果这个距离比次短路还长，丢弃
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continue;
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}
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int sz=G[v].size();
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for(int i=0;i<sz;i++)
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{
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edge& e=G[v][i];
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int dd=d+e.dist;
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if(dis[e.to]>dd)
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{
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/// 如果最短路比新合成距离长，交换(?)
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swap(dis[e.to],dd);
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bus.push(PR(dis[e.to],e.to));
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}
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/// 现在一定有: dd>dis[e.to]
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if(second_dis[e.to] > dd && dis[e.to] < dd)
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{
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second_dis[e.to]=dd;
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bus.push(PR(second_dis[e.to],e.to));
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}
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}
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}
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printf("%d\n",second_dis[N-1]);
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}
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int main()
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{
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scanf("%d %d",&N,&R);
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for(int i=0;i<R;i++)
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{
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int A,B,D;
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scanf("%d %d %d",&A,&B,&D);
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G[A-1].push_back(edge(B-1,D));
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G[B-1].push_back(edge(A-1,D));
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}
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solve();
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return 0;
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}
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