枚举最短路径+SPFA

Harry Potter and the Final Battle

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Description

The final battle is coming. Now Harry Potter is located at city 1, and Voldemort is located at city n. To make the world peace as soon as possible, Of course, Harry Potter will choose the shortest road between city 1 and city n. But unfortunately, Voldemort is so powerful that he can choose to destroy any one of the existing roads as he wish, but he can only destroy one. Now given the roads between cities, you are to give the shortest time that Harry Potter can reach city n and begin the battle in the worst case.

 

Input

First line, case number t (t<=20).

Then for each case: an integer n (2<=n<=1000) means the number of city in the magical world, the cities are numbered from 1 to n. Then an integer m means the roads in the magical world, m (0< m <=50000). Following m lines, each line with three integer u, v, w (u != v,1 <=u, v<=n, 1<=w <1000), separated by a single space. It means there is a bidirectional road between u and v with the cost of time w. There may be multiple roads between two cities.

 

Output

Each case per line: the shortest time to reach city n in the worst case. If it is impossible to reach city n in the worst case, output “-1”.

 

Sample Input

3
4
4
1 2 5
2 4 10
1 3 3
3 4 8
3
2
1 2 5
2 3 10
2
2
1 2 1
1 2 2

 

Sample Output

15
-1
2

#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
#include<queue>
using namespace std;

const int N=1010;
const int M=100010;
const int INF=0xffffff;

struct Edge
{
    int u;
    int to;
    int w;
    int flag;
    int next;
} e[M];

int head[N];
int dist[N];
int path[N];
int inq[N];
int n,m,cnt,flag;

void AddEdge(int u,int v,int w)
{
    e[cnt].u=u;
    e[cnt].to=v;
    e[cnt].w=w;
    e[cnt].flag=1;
    e[cnt].next=head[u];
    head[u]=cnt++;
}

int SPFA(int s)
{
    queue<int>Q;
    for(int i=1; i<=n; i++)
    {
        dist[i]=INF;
        inq[i]=0;
    }
    dist[s]=0;
    inq[s]=1;
    Q.push(s);
    while(!Q.empty())
    {
        int u=Q.front();
        Q.pop();
        inq[u]=0;
        for(int j=head[u]; j!=-1; j=e[j].next)
        {
            int x=e[j].to;
            if(e[j].flag&&dist[x]>dist[u]+e[j].w)
            {
                dist[x]=dist[u]+e[j].w;
                if(!flag)
                    path[x]=j;
                if(!inq[x])
                {
                    Q.push(x);
                    inq[x]=1;
                }
            }
        }
    }
    return dist[n];
}

int main()
{
    //freopen("C:\\Users\\Administrator\\Desktop\\kd.txt","r",stdin);
    int t;
    scanf("%d",&t);
    while(t--)
    {
        cnt=flag=0;
        memset(head,-1,sizeof(head));
        scanf("%d%d",&n,&m);
        while(m--)
        {
        	int u,v,w;
            scanf("%d%d%d",&u,&v,&w);
            AddEdge(u,v,w);
            AddEdge(v,u,w);
        }
        memset(path,-1,sizeof(path));
        SPFA(1);
        flag=1;
        int i=n,j=-1;
        int res=-1;
        while(path[i]!=-1)
        {
            j=path[i];
            e[j].flag=e[j+1].flag=0;
            int tmp=SPFA(1);
            e[j].flag=e[j+1].flag=1;
            if(tmp>res)
                res=tmp;
            i=e[j].u;
        }
        if(res<INF)
            printf("%d\n",res);
        else
            puts("-1");
    }
}