POJ 3268 Silver Cow Party

#include <iostream>
#include <cstring>
#define INF 999999999
using namespace std;

int N,M,X;
int e[1001][1001];//邻接矩阵储存地图 
int m[1001][1001];//转置后的矩阵 
int dis1[1001];//第一次求最短路径 
int book[1001];
int dis2[1001];//第二次求最短路径 

int main()
{
    //freopen("data.txt","r",stdin); 
    int a,b,c;
    int temp;
    int step;
    int res; 
    while(cin>>N>>M>>X)
    {
        //第一步 
        res = 0;
        //初始化
        memset(book,0,sizeof(book)); 
        
        for(int i=1;i<=N;i++)
            dis1[i] = INF;
        
        for(int i=1;i<=N;i++)
            for(int j=1;j<=N;j++)
            {
                if(j==i)
                    e[i][j] = 0;
                else
                    e[i][j] = INF;
            }
    
        for(int i=1;i<=M;i++)
        {
            cin>>a>>b>>c;    
            e[a][b] = c;
        }
        
        //初始化dis1数组
        for(int i=1;i<=N;i++)
        {
            dis1[i] = e[X][i];
        }
        
        //dijkstra算法核心 
        book[X] = 1;
        temp = X;
        for(int i=1;i<=N-1;i++)
        {
            step = INF;
            for(int j=1;j<=N;j++)
            {
                if(book[j]==0&&step>dis1[j])
                {
                    step = dis1[j];
                    temp = j;
                }
            }

            book[temp] = 1;
            
            for(int k=1;k<=N;k++)
            {
                if(dis1[k]>dis1[temp]+e[temp][k])
                    dis1[k] = dis1[temp]+e[temp][k];
            }
        }
         
         //第二步 
        //矩阵反转 
        for(int i=1;i<=N;i++)
        {
            for(int j=1;j<=N;j++)
            {
                m[i][j] = e[j][i];
            }
         } 
         
         
         //初始化dis2 
         for(int i=1;i<=N;i++)
            dis2[i] = INF;
        for(int i=1;i<=N;i++)
        {
            dis2[i] = m[X][i];
        }
        
        memset(book,0,sizeof(book));
        
        //dijkstra算法核心 
        book[X] = 1;
        temp = X;
        for(int i=1;i<=N-1;i++)
        {
            step = INF;
            for(int j=1;j<=N;j++)
            {
                if(book[j]==0&&step>dis2[j])
                {
                    step = dis2[j];
                    temp = j;
                }
            }
            
            book[temp] = 1;
            
            for(int k=1;k<=N;k++)
            {
                if(dis2[k]>dis2[temp]+m[temp][k])
                    dis2[k] = dis2[temp]+m[temp][k];
            }
        }
        
        
        //第三步，进行相加比较得出结果 
        for(int i=1;i<=N;i++)
        {
            if(dis1[i]==INF||dis2[i]==INF)
                continue;
            else
                res = max(dis1[i]+dis2[i],res);
        }
        
        cout<<res<<endl;
        
    }
    return 0;
}