POJ_3463_Sightseeing(最短路/次短路条数)

Sightseeing

Description

Tour operator Your Personal Holiday organises guided bus trips across the Benelux. Every day the bus moves from one city
S to another city F. On this way, the tourists in the bus can see the sights alongside the route travelled. Moreover, the bus makes a number of stops (zero or more) at some beautiful cities, where the tourists get out to see the local sights.

Different groups of tourists may have different preferences for the sights they want to see, and thus for the route to be taken from
S to F. Therefore, Your Personal Holiday wants to offer its clients a choice from many different routes. As hotels have been booked in advance, the starting city
S and the final city F, though, are fixed. Two routes from
S to F are considered different if there is at least one road from a city
A to a city B which is part of one route, but not of the other route.

There is a restriction on the routes that the tourists may choose from. To leave enough time for the sightseeing at the stops (and to avoid using too much fuel), the bus has to take a short route from
S to F. It has to be either a route with minimal distance, or a route which is one distance unit longer than the minimal distance. Indeed, by allowing routes that are one distance unit longer, the tourists may have more choice than by restricting
them to exactly the minimal routes. This enhances the impression of a personal holiday.

For example, for the above road map, there are two minimal routes from S = 1 to
F = 5: 1 → 2 → 5 and 1 → 3 → 5, both of length 6. There is one route that is one distance unit longer: 1 → 3 → 4 → 5, of length 7.

Now, given a (partial) road map of the Benelux and two cities S and
F, tour operator Your Personal Holiday likes to know how many different routes it can offer to its clients, under the above restriction on the route length.

Input

The first line of the input file contains a single number: the number of test cases to follow. Each test case has the following format:

• One line with two integers N and M, separated by a single space, with 2 ≤
  N ≤ 1,000 and 1 ≤ M ≤ 10, 000: the number of cities and the number of roads in the road map.
• M lines, each with three integers A, B and L, separated by single spaces, with 1 ≤
  A, B ≤ N, A ≠ B and 1 ≤ L ≤ 1,000, describing a road from city
  A to city B with length L.

  The roads are unidirectional. Hence, if there is a road from A to
  B, then there is not necessarily also a road from B to A. There may be different roads from a city
  A to a city B.

• One line with two integers S and F, separated by a single space, with 1 ≤
  S, F ≤ N and S ≠ F: the starting city and the final city of the route.

  There will be at least one route from S to F.

Output

For every test case in the input file, the output should contain a single number, on a single line: the number of routes of minimal length or one distance unit longer. Test cases are such, that this number is at most 10⁹ = 1,000,000,000.

Sample Input

2
5 8
1 2 3
1 3 2
1 4 5
2 3 1
2 5 3
3 4 2
3 5 4
4 5 3
1 5
5 6
2 3 1
3 2 1
3 1 10
4 5 2
5 2 7
5 2 7
4 1

Sample Output

3
2

Hint

The first test case above corresponds to the picture in the problem description.

题意：给你一个有向图，如果次短路长度刚好比最短路大1，那么输出最短路条数与次短路条数之和，否者直接输出最短路的条数。

分析：模板题。代码有注释哟~~~

题目链接：http://poj.org/problem?id=3463

代码清单：

#include<map>
#include<queue>
#include<stack>
#include<ctime>
#include<cctype>
#include<cstdio>
#include<cstring>
#include<string>
#include<iostream>
#include<algorithm>
using namespace std;
typedef long long ll;
const int maxn = 1000 + 5;
const int max_dis = 1e9;

struct Edge{
    int to;
    int dis;
    Edge(int to,int dis){
        this -> to = to;
        this -> dis = dis;
    }
};

struct Node{
    int d;
    int v;
    int pose;
    friend bool operator<(Node x,Node y){
        if(x.d==y.d) return x.v>y.v;
        return x.d>y.d;
    }
};

int T,N,M;
int a,b,c,S,F;
bool vis[maxn][2];
int dist[maxn][2];
int degree[maxn][2];
vector<Edge>G[maxn];

void dij(int s,int e){

    for(int i=0;i<maxn;i++){
        dist[i][0]=dist[i][1]=max_dis;
        degree[i][0]=degree[i][1]=0;
        vis[i][0]=vis[i][1]=false;
    }
    priority_queue<Node>q;
    while(q.size()) q.pop();
    Node p,w;
    p.d=0; p.v=s; p.pose=0;
    degree[p.v][p.pose]=1;
    q.push(p);
    while(q.size()){
        p=q.top(); q.pop();
        if(vis[p.v][p.pose]) continue;
        vis[p.v][p.pose]=true;
        for(int i=0;i<G[p.v].size();i++){

            Edge& e=G[p.v][i];
            if(!vis[e.to][0]&&p.d+e.dis<dist[e.to][0]){  //找到一条比当前最短路更短的路
                if(dist[e.to][0]!=max_dis){              //作为次短路，入队
                    w.v=e.to; w.d=dist[e.to][0]; w.pose=1;
                    dist[e.to][1]=dist[e.to][0];
                    degree[e.to][1]=degree[e.to][0];
                    q.push(w);
                }
                w.v=e.to; w.d=p.d+e.dis; w.pose=0;       //更新最短路，入队
                dist[e.to][0]=w.d; degree[e.to][0]=degree[p.v][p.pose];
                q.push(w);
            }

            else if(!vis[e.to][0]&&p.d+e.dis==dist[e.to][0]){ //找到一条相同距离的最短路，更新条数，不入队
                degree[e.to][0]+=degree[p.v][p.pose];
            }

            else if(!vis[e.to][1]&&p.d+e.dis<dist[e.to][1]){ //找到一条比当前次短路更短的路，不可以更新最短路，可更新次短路，入队
                w.v=e.to; w.d=p.d+e.dis; w.pose=1;
                dist[e.to][1]=w.d; degree[e.to][1]=degree[p.v][p.pose];
                q.push(w);
            }

            else if(!vis[e.to][1]&&p.d+e.dis==dist[e.to][1]){ //找到一条相同距离的次短路，更新条数，不入队
                degree[e.to][1]+=degree[p.v][p.pose];
            }
        }
    }
}

int main(){
    scanf("%d",&T);
    while(T--){
        for(int i=0;i<maxn;i++) G[i].clear();
        scanf("%d%d",&N,&M);
        for(int i=0;i<M;i++){
            scanf("%d%d%d",&a,&b,&c);
            G[a].push_back(Edge(b,c));
        }
        scanf("%d%d",&S,&F);
        dij(S,F);
        if(dist[F][0]+1==dist[F][1])
            printf("%d\n",degree[F][0]+degree[F][1]);
        else
            printf("%d\n",degree[F][0]);
    }return 0;
}