poj 2391 Ombrophobic Bovines(最大流+floyd+二分)

view code#include <cstdio>
#include <algorithm>
#include <cstring>
#include <iostream>
#include <queue>
using namespace std;
typedef long long ll;
const ll INF = 1LL<<60;
const int inf = 1<<30;
const int N = 500;
int n, F, P, pre[N], cur[N];
int s, t, d[N];
ll dis[N][N];

struct om
{
    int num, cap;
}loc[N];

struct edge
{
    int u, v, cap, flow, next;
    edge(int u, int v, int cap, int flow, int next):u(u), v(v), cap(cap), flow(flow), next(next) {}
    edge() {}
}e[N*N*4];
int ecnt;

void floyd()
{
    for(int k=1; k<=n; k++)
        for(int i=1; i<=n; i++) if(dis[i][k]!=INF)
            for(int j=1; j<=n; j++)
                if(dis[k][j]!=INF && dis[i][j]>dis[i][k] + dis[k][j])
                    dis[i][j] = dis[i][k] + dis[k][j];
}

void addedge(int u, int v, int w)
{
    e[ecnt] = edge(u, v, w, 0, pre[u]);
    pre[u] = ecnt++;
    e[ecnt] = edge(v, u, 0, 0, pre[v]);
    pre[v] = ecnt++;
}

bool vis[N<<1];
bool BFS()
{
    memset(vis, 0 ,sizeof(vis));
    queue<int > q;
    q.push(s);
    d[s] = 0;
    vis[s] = 1;
    while(!q.empty())
    {
        int x = q.front(); q.pop();
        for(int i = pre[x]; ~i; i=e[i].next)
        {
            int v = e[i].v;
            if(!vis[v] && e[i].cap>e[i].flow)
            {
                vis[v] = 1;
                d[v] = d[x] + 1;
                q.push(v);
            }
        }
    }
    return vis[t];
}

int DFS(int x, int c)
{
    if(x==t || c==0) return c;
    int flow = 0, f;
    for(int &i=cur[x]; ~i; i=e[i].next)
    {
        int v = e[i].v;
        if(d[x]+1==d[v] && (f=DFS(v,min(c,e[i].cap-e[i].flow)))>0)
        {
            e[i].flow += f;
            e[i^1].flow -=f;
            flow += f;
            c -= f;
            if(c==0) break;
        }
    }
    return flow;
}

ll Maxflow(int s, int t)
{
    int flow = 0;
    while(BFS())
    {
        for(int i=s; i<=t; i++) cur[i] = pre[i];
        flow += DFS(s, inf);
    }
    return flow;
}

bool is_ok(ll m, int sum)
{
    s = 0, t = n*2+1;
    memset(pre, -1, sizeof(pre));
    ecnt = 0;
    for(int i=1; i<=n; i++)
    {
        addedge(s, i, loc[i].num);
        addedge(i+n, t, loc[i].cap);
    }

    for(int i=1; i<=n; i++)
        for(int j=1; j<=n; j++) if(dis[i][j]<=m)
                addedge(i, j+n, inf);

    return Maxflow(s,t)>=sum;
}

ll solve()
{
    int sumn = 0, sumc = 0;
    for(int i=1; i<=n; i++)
    {
        scanf("%d%d", &loc[i].num, &loc[i].cap);
        sumn += loc[i].num;
        sumc += loc[i].cap;
    }
    int u, v, w;
    for(int i=1; i<=n; i++)
    {
        for(int j=1; j<=n; j++) dis[i][j] = INF;
        dis[i][i] = 0;
    }
    for(int i=0; i<P; i++)
    {
        scanf("%d%d%d", &u, &v, &w);
        if(w<dis[u][v]) dis[u][v] = dis[v][u] = w;
    }
    if(sumn > sumc) return -1;
    floyd();

    ll l = 0, r = 0, ans = -1;
    for(int i=1; i<n; i++)
    {
        for(int j=i+1; j<=n; j++)
           if(r<dis[i][j] && dis[i][j]!=INF) r = dis[i][j];
    }
    while(l<=r)
    {
        ll mid = (l+r)>>1;
        if(is_ok(mid, sumn)) ans = mid, r = mid - 1;
        else l = mid + 1;
//        printf("ans = %d\n", ans);
    }
    return ans;
}

int main()
{
//    freopen("in", "r", stdin);
    while(scanf("%d%d", &n, &P)>0) cout<<solve()<<endl;
    return 0;
}