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最新推荐文章于 2018-11-27 09:27:56 发布

原创最新推荐文章于 2018-11-27 09:27:56 发布 · 645 阅读

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#sap #struct

本文详细介绍了在使用Floyd算法求最短路径时遇到的INF问题，并提供了一个简单的解决方法，通过将INF替换为20000000000LL，使得程序能够正确运行并得到预期结果。

还在wa 不知道为啥没过..

一会再看吧

此题需要注意在floyd时的inf 。。另取一个inf 换成20000000000ll就过了

#include <cstdio>
#include <cstring>
#include <iostream>
using namespace std;
const int MAXN=2000;

const int inf = 0x7fffffff;

const int s = 0;

struct edge{
int v,next,w;
}edge[100000];

int head[2*MAXN],cnt;//for sap

void addedge(int u, int v, int w)
{
     edge[cnt].v = v;
     edge[cnt].w = w;
     edge[cnt].next = head[u];
     head[u] = cnt++;
     edge[cnt].v = u;
     edge[cnt].w = 0;
     edge[cnt].next = head[v];
     head[v] = cnt++;
}

int sap(int t)
{
    int pre[2*MAXN],cur[2*MAXN],dis[2*MAXN],gap[2*MAXN]; //gap标记断层
    int flow = 0 , aug = inf ,u;
    bool flag;
    for (int i = 0 ; i <= t ; ++i)
    {
        cur[i] = head[i];
        gap[i] = dis[i] = 0;
    }
    gap[s] = t+1;
    u = pre[s] = s;
    while (dis[s] <= t)
    {
          flag = 0 ;
          for (int &j = cur[u] ; ~j ; j = edge[j].next)
          {
              int v = edge[j].v;
              if (edge[j].w > 0 && dis[u] == dis[v]+1)
              {
                   flag = 1;
                   if( edge[j].w < aug )aug = edge[j].w;
                   pre[v] = u;
                   u = v;
                   if (u == t)
                   {
                       flow += aug;
                       while (u != s)
                       {
                             u = pre[u];
                             edge[cur[u]].w -= aug;
                             edge[cur[u]^1].w += aug;
                       }
                       aug = inf;
                   }
                   break;
              }
          }
          if ( flag )continue ;
          int mindis = t+1;
          for (int j = head[u]; ~j ; j = edge[j].next)
          {
              int v = edge[j].v;
              if (edge[j].w > 0 && dis[v] < mindis)
              {
                 mindis = dis[v];
                 cur[u] = j;
              }
          }
          if(--gap[dis[u]] == 0)break;
          gap[ dis[u] = mindis+1 ]++;
          u = pre[u];
    }
    return flow;
}

void init ()
{
memset (head , -1 , sizeof(head));
cnt=0;
}

long long map[302][302];
long long cap[302];
long long cow[302];

void floyd ( int n )
{
    for ( int k = 0 ; k < n ; k ++ )
        for ( int i = 0 ; i < n; i ++ )
            for ( int j = 0 ; j < n; j ++ )
                if ( map[i][j] > map[i][k] + map[k][j] )
                    map[i][j] = map[i][k] + map[k][j];
}

int main()
{
    int f, p;
    //freopen ( "out.txt" , "r" , stdin );
    //freopen ( "myrun.txt" , "w" , stdout);
    //cout << inf << endl;
    while ( cin >> f >> p )
    {
        init ();
        long long a, b , w;
        long long sum = 0;
        for ( int i = 0 ; i < f ; i ++ )
            for ( int j = 0 ; j < f ; j ++ )
                map[i][j] = inf;
        for ( int i = 0 ; i < f ; i ++ )
        {
            cin >> a >> b;
            cow[i] = a;
            cap[i] = b;
            sum += a;
        }
        for ( int i = 0 ; i < p ; i ++)
        {
            cin >> a >> b >> w;
            a--, b--;
            if ( map[a][b] > w )
                map[a][b] = map[b][a] = w;
        }
        for ( int i = 0 ; i < f ; i ++)
            map[i][i] = 0;
        floyd ( f );
        long long ans = inf;
        long long begin = 0 , end = inf;
        for ( int i = 0 ; i < f ; i ++ )
            for ( int j = 0 ; j < f ; j ++ )
                if ( end < map[i][j] )
                    end = map[i][j];
//        for ( int i =0 ; i < f ;i ++ )
//        {
//            for ( int j =0 ; j < f ; j++ )
//                cout << map[i][j] << " " ;
//            cout << endl;
//        }
        //end += 100;
        while ( begin < end )
        {
            long long mid = ( begin + end )/2;
            init ();
            for ( int i = 0 ; i < f ; i ++)
                for ( int j = 0 ; j < f ; j ++ )
                    if ( map[i][j] <= mid && i != j )
                    {
                        addedge ( i+1 , 250+j , inf );
                    }
            for ( int i = 0 ; i < f ; i ++)
                addedge ( i+1, 250+i , inf );
            for ( int i = 0 ; i < f ; i ++)
                addedge ( 0 , i+1 , cow[i] );
            for ( int i = 0 ; i < f ; i ++ )
                addedge ( 250+i , 455 , cap[i] );
            long long tmp = sap ( 455 );
            //cout << tmp << endl;
            if ( sum == tmp )
            {
                if ( ans > mid )
                    ans = mid;
                end = mid;
            }
            else
                begin = mid+1 ;
        }
        if ( end > 2000000000 )
            cout << "-1" << endl;
        else if ( ans < inf )
            cout << ans << endl;
        else
            cout <<"-1" << endl;
    }
    return 0;
}