Hdu2680 最短路

给定一个有向图，多个起点，一个终点，求起点到终点的最短路。

1.可以加一个点，使其与那些起点的距离为0

2.将图反着来建，然后在所有点找出最小的

方案一：

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <map>
#define MaxInt 0x3f3f3f3f
using namespace std;
int Map[1005][1005];
int vis[1005];
int low[1005];
int p[1005];
int n,m,s;
int k;
void work()
{
    int pos = 0,Min;
    memset(vis,0,sizeof(vis));
    for(int i = 0; i <= n; i++)
        low[i] = Map[pos][i];
    vis[pos] = 1;
    for(int i = 0; i < n; i++)
    {

        Min = MaxInt;
        for(int j = 0; j <= n; j++)
            if(!vis[j] && low[j] < Min)
            {
                pos = j;
                Min = low[j];
            }
        if(Min == MaxInt)
            break;
        vis[pos] = 1;
        for(int j = 0; j <= n; j++)
            if(!vis[j] && low[j] > low[pos]+Map[pos][j])
                low[j] = low[pos]+Map[pos][j];
    }
    if(low[s] < MaxInt)
        printf("%d\n",low[s]);
    else
        printf("-1\n");
}

int main()
{
    int a,b,c;
    while(~scanf("%d%d%d",&n,&m,&s))
    {
        for(int i = 0; i <= n; i++)
        {
            Map[i][i] = 0;
            for(int j = 0; j <= n; j++)
                Map[i][j] = MaxInt;
        }
        for(int i = 1; i <= m; i++)
        {
            scanf("%d%d%d",&a,&b,&c);
            if(Map[a][b] > c)
                Map[a][b]=c;
        }
        int all,x;
        scanf("%d",&all);
        for(int i = 1; i <= all; i++){    //加入0点，并使其为起始点
            scanf("%d",&x);
            Map[0][x] = 0;
        }
        work();
    }
    return 0;
}

　

方案二：

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#define MaxInt 0x3f3f3f3f
using namespace std;
int Map[1005][1005];
int vis[1005];
int low[1005];
int p[1005];
int n,m,s;
int k;
void work()
{
    int pos = s,Min;
    memset(vis,0,sizeof(vis));
    for(int i = 0; i <= n; i++)
        low[i] = Map[pos][i];
    vis[pos] = 1;
    for(int i = 0; i < n; i++)
    {

        Min = MaxInt;
        for(int j = 0; j <= n; j++)
            if(!vis[j] && low[j] < Min)
            {
                pos = j;
                Min = low[j];
            }
        if(Min == MaxInt)
            break;
        vis[pos] = 1;
        for(int j = 0; j <= n; j++)
            if(!vis[j] && low[j] > low[pos]+Map[pos][j])
                low[j] = low[pos]+Map[pos][j];
    }
}

int main()
{
    int a,b,c;
    while(~scanf("%d%d%d",&n,&m,&s))
    {
        for(int i = 0; i <= n; i++)
        {
            Map[i][i] = 0;
            for(int j = 0; j <= n; j++)
                Map[i][j] = MaxInt;
        }
        for(int i = 1; i <= m; i++)
        {
            scanf("%d%d%d",&a,&b,&c);
            if(Map[b][a] > c)     //反着建图
                Map[b][a]=c;
        }
        work();
        int num,x;
        int MinInt = MaxInt;
        scanf("%d",&num);
        for(int i = 1;i <= num;i++)
        {
            scanf("%d",&x);
            if(low[x] < MinInt)         //找出最短的路
                MinInt = low[x];
        }
        if(MinInt < MaxInt)
            printf("%d\n",MinInt);
        else
            printf("-1\n");
    }
    return 0;
}