HDU1595

SPFA

题意：给定一张图，从1到n。

求1到n的最短路，但是可能存在某条路被阻断了，即为inf了。

这条路必定是存在最短路中。所以枚举最短路中的路径。。。。

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#include<queue>
#include<algorithm>
using namespace std;
const int maxn = 1005;
const int maxm = 1005*500;
const int inf = 9999999;
struct node{
    int u,val,next;
}edge[ maxm*2 ];
int head[ maxn ],cnt;
int dis[ maxn ],vis[ maxn ],path[ maxn ];
int n,m,CNT;
struct node22{
    int x,y;
}road[ maxn ];
int mat[ maxn ][ maxn ];
void init(){
    cnt=0;
    CNT=0;
    memset( path,-1,sizeof(path) );
    memset( head,-1,sizeof( head ));
}
void addedge( int a,int b,int c ){
    edge[ cnt ].u=b;
    edge[ cnt ].val=c;
    edge[ cnt ].next=head[ a ];
    head[ a ]=cnt++;
}
void changeit(const int &x, const int &y, int va)  
{  
    for( int i=head[x];i!=-1;i=edge[i].next ){
        if( edge[i].u==y ){
            edge[i].val=va;
        }
    }
}  

void spfa( int s ){
    for( int i=1;i<=n;i++ ){
        dis[i]=inf;
        vis[i]=0;
        //path[i]=s;
    }
    queue<int>q;
    while( !q.empty() )
        q.pop();
    vis[s]=1;
    dis[s]=0;
    q.push( 1 );
    while( !q.empty() ){
        int now=q.front();
        q.pop();
        vis[ now ]=0;
        for( int i=head[ now ];i!=-1;i=edge[i].next ){
            int next=edge[i].u;
            if( dis[next]>dis[now]+edge[i].val ){
                dis[next]=dis[now]+edge[i].val;
                path[next]=now;
                if( vis[next]==0 ){
                    vis[next]=1;
                    q.push(next);
                }
            }
        }
    }
    return ;
}

int main(){
    while( scanf("%d%d",&n,&m)!=EOF ){
        int a,b,c;
        init();
        while( m-- ){
            scanf("%d%d%d",&a,&b,&c);
            addedge( a,b,c );
            addedge( b,a,c );
            mat[a][b]=mat[b][a]=c;
        }
        spfa( 1 );
        for( int i=n;i!=1;i=path[i] ){
            road[ CNT ].x=i;
            road[ CNT ].y=path[i];
            CNT++;
        }
        int ans=0;
        for(int i = 0; i < CNT; i++){  
            changeit(road[i].x, road[i].y, inf);  
            changeit(road[i].y, road[i].x, inf);   
            spfa(1);  
            if(dis[n] > ans&&dis[n]!=inf) ans = dis[n];  
            changeit(road[i].x, road[i].y, mat[road[i].x][road[i].y]);  
            changeit(road[i].y, road[i].x, mat[road[i].x][road[i].y]);  
        }  
        printf("%d\n", ans);
    }
    return 0;
}