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  • poj 2983 差分约束

    思路:

    设dis[i]为标号为i的点到0号点的距离。对于P A B X,我们能得到等式dis[a]-dis[b]=x,那么可以化为两个不等式dis[a]-dis[b]>=x和dis[b]-dis[a]>=-x。这样就可以建两条边。V A B的话,我们知道dis[a]-dis[b]>=1,可以建一条边。这些边建起来后,图可能是一个离散的图,那么我们就定义一个超级源点连接所有的点,权值为0.进行求最长路时,只要判断是否有正圈存在,正圈的含义是绕着这个圈使每个点的dis值不断增大。用bellman-ford算法就行。还有一个笨的方法,其实是卡数据的,我们就用spfa求最长路,若循环次数超过一定,我们就认为有正圈存在。

    这个是用bellman_ford做的:

    #include<iostream>
    #include<cstdio>
    #include<cstring>
    #include<algorithm>
    #include<queue>
    #define inf 1<<30
    #define Maxn 10010
    #define Maxm 500000
    using namespace std;
    int dis[Maxn],vi[Maxn],index[Maxn],e,Que[1000010],num=0,n;
    struct Edge{
        int to,next,val,from;
    }edge[Maxm];
    void init()
    {
        int i,j;
        for( i=0;i<=Maxn-1;i++)
            dis[i]=-inf;
        memset(vi,0,sizeof(vi));
        memset(index,-1,sizeof(index));
        e=0;
        num=0;
    }
    void addedge(int from,int to,int val)
    {
        edge[e].from=from;
        edge[e].to=to;
        edge[e].val=val;
        edge[e].next=index[from];
        index[from]=e++;
    }
    int bellman_ford()
    {
        int i,j,temp,flag;
        for(i=1;i<=n;i++)
        {
            flag=1;
            for(j=0;j<e;j++)
            {
                temp=edge[j].from;
                if(dis[temp]+edge[j].val>dis[edge[j].to])
                {
                    dis[edge[j].to]=dis[temp]+edge[j].val;
                    flag=0;
                }
            }
            if(flag)
                return 1;
        }
        return 0;
    }
    int main()
    {
        int i,j,a,b,c,m;
        char str[2];
        while(scanf("%d%d",&n,&m)!=EOF)
        {
            init();
            //cout<<"ok"<<endl;
            for(i=1;i<=m;i++)
            {
                scanf("%s",&str);
                if(str[0]=='P')
                {
                    scanf("%d%d%d",&a,&b,&c);
                    addedge(b,a,c);
                    addedge(a,b,-c);
                }
                else
                {
                    scanf("%d%d",&a,&b);
                    addedge(b,a,1);
                }
            }
            for(i=1;i<=n;i++)
                addedge(0,i,0);
            if(bellman_ford())
                printf("Reliable
    ");
            else
                printf("Unreliable
    ");
        }
        return 0;
    }
    View Code

    给个卡数据的spfa:

    #include<iostream>
    #include<cstdio>
    #include<cstring>
    #include<algorithm>
    #include<queue>
    #define inf 1<<30
    #define Maxn 10010
    #define Maxm 500000
    using namespace std;
    int dis[Maxn],vi[Maxn],index[Maxn],e,Que[1000010],num=0;
    struct Edge{
        int to,next,val;
    }edge[Maxm];
    void init()
    {
        int i,j;
        for( i=0;i<=Maxn-1;i++)
            dis[i]=-inf;
        memset(vi,0,sizeof(vi));
        memset(index,-1,sizeof(index));
        e=0;
        num=0;
    }
    void addedge(int from,int to,int val)
    {
        edge[e].from=from;
        edge[e].to=to;
        edge[e].val=val;
        edge[e].next=index[from];
        index[from]=e++;
    }
    int spfa()
    {
        int i,j,temp,head,rear;
        head=rear=0;
        Que[head++]=0;
        dis[0]=0;
        //cout<<maxn<<endl;
        while(head!=rear)
        {
            temp=Que[rear++];
            //cout<<temp<<endl;
            vi[temp]=0;
            for(i=index[temp];i!=-1;i=edge[i].next)
            {
                int now=edge[i].to;
                if(dis[now]<dis[temp]+edge[i].val)
                {    
                    num++;
                    if(num>500000)
                        return 0;
                    if(edge[i].val<0)
                    {
                        dis[temp]+edge[i].val+graphic[now][temp]
                    }
                    dis[now]=dis[temp]+edge[i].val;
                    if(!vi[now])
                        Que[head++]=now;
                    vi[now]=1;
                }
            }
        }
        return 1;
    }
    int main()
    {
        int i,j,n,a,b,c,m;
        char str[2];
        while(scanf("%d%d",&n,&m)!=EOF)
        {
            init();
            //cout<<"ok"<<endl;
            for(i=1;i<=m;i++)
            {
                scanf("%s",&str);
                if(str[0]=='P')
                {
                    scanf("%d%d%d",&a,&b,&c);
                    addedge(b,a,c);
                    addedge(a,b,-c);
                }
                else
                {
                    scanf("%d%d",&a,&b);
                    addedge(b,a,1);
                }
            }
            for(i=1;i<=n;i++)
                addedge(0,i,0);
            if(spfa())
                printf("Reliable
    ");
            else
                printf("Unreliable
    ");
        }
        return 0;
    }
    View Code
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  • 原文地址:https://www.cnblogs.com/wangfang20/p/3197606.html
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