Description
Bessie has moved to a small farm and sometimes enjoys returning to visit one of her best friends. She does not want to get to her old home too quickly, because she likes the scenery along the way. She has decided to take the second-shortest rather than the shortest path. She knows there must be some second-shortest path.
The countryside consists of R (1 ≤ R ≤ 100,000) bidirectional roads, each linking two of the N (1 ≤ N ≤ 5000) intersections, conveniently numbered 1..N. Bessie starts at intersection 1, and her friend (the destination) is at intersection N.
The second-shortest path may share roads with any of the shortest paths, and it may backtrack i.e., use the same road or intersection more than once. The second-shortest path is the shortest path whose length is longer than the shortest path(s) (i.e., if two or more shortest paths exist, the second-shortest path is the one whose length is longer than those but no longer than any other path).
Input
Line 1: Two space-separated integers: N and R
Lines 2..R+1: Each line contains three space-separated integers: A, B, and D that describe a road that connects intersections A and B and has length D (1 ≤ D ≤ 5000)
Output
Line 1: The length of the second shortest path between node 1 and node N
Sample Input
4 4
1 2 100
2 4 200
2 3 250
3 4 100
Sample Output
450
Hint
Two routes: 1 -> 2 -> 4 (length 100+200=300) and 1 -> 2 -> 3 -> 4 (length 100+250+100=450)
分析：
次短路不可能与最短路完全重合，那么就一定会有一段比较绕路。绕路的地方不可能超过两处，那样就有一条路短于这条路而长于最短路，矛盾。 因此可以对所有的边加以枚举。
首先用Dijkstra或SPFA以原点和终点为源分别做一次单源最短路，并把答案存在dist_0和dist_n两个数组中。那么，对于任何一条边(i, j)，
下面的二者之一就有可能是次短路的长：
dist_0[i] + len(i, j) + dist_n[j] 和 dist_0[j] + len(i, j) + dist_n[i]
注意，如果其中一个的长度等于最短路的长度（即dist_n[0]），就一定不能选，因为这违反次短路的定义。两个都要枚举，因为可能有其中一个等于最短路的长，如果只取较小的值另外一个就废掉了。
#include<stdio.h>
#include<iostream>
#include<queue>
#include<string.h>
const int INF=0x3f3f3f3f;
using namespace std;
int n,m,cnt;
struct Node
{
int to,val;
int next;
} node[200100];
int head[5009],vis[5009];//寻找头结点，在找最短路的时候这个点有没有访问过
int dis_1[5009],dis_n[5009];//到1号节点的最短路，到n号节点的最短路
void add(int a,int b,int w)//将边的信息采用头插法保存下来
{
node[cnt].to=b;
node[cnt].val=w;
node[cnt].next=head[a];
head[a]=cnt;
cnt++;
}
void spfa(int st,int dis[])//spfa求最短路的模板
{
memset(vis,0,sizeof(vis));
queue<int>q;
dis[st]=0;
vis[st]=1;
q.push(st);
while(!q.empty())
{
int x=q.front();
q.pop();
vis[x]=0;
for(int i=head[x];i!=-1;i=node[i].next)
{
int y=node[i].to;
if(dis[y]>dis[x]+node[i].val)
{
dis[y]=dis[x]+node[i].val;
if(vis[y]==0)
{
vis[y]=1;
q.push(y);
}
}
}
}
}
int main()
{
int a,b,w;
while(~scanf("%d%d",&n,&m))
{
for(int i=0;i<=n;i++)//初始化
{
head[i]=-1;
dis_1[i]=dis_n[i]=INF;
}
cnt=0;//代表边数
for(int i=0; i<m; i++)
{
scanf("%d%d%d",&a,&b,&w);
add(a,b,w);
add(b,a,w);
}
spfa(1,dis_1);
spfa(n,dis_n);
int ans=INF;
//一条边一条边的通过绕路寻找次短路
for(int i=1;i<=n;i++)
{
for(int j=head[i];j!=-1;j=node[j].next)
{
b=node[j].to;
w=node[j].val;
int temp=dis_1[i]+w+dis_n[b];
if(temp>dis_1[n]&&ans>temp)
{
ans=temp;
}
}
}
printf("%d
",ans);
}
return 0;
}