Destroying the bus stations HDU

题意：

　　就是求最小割点

解析：

　　正向一遍spfa 反向一遍spfa  然后遍历每一条边，对于当前边 如果dis1[u] + dis2[v] + 1 <= k 那么就把这条边加入到网络流图中，

　　每个点拆点 边权为1

　　跑最大流即可

代码还是改的那一题。。。

　　

#include <iostream>
#include <cstring>
#include <cstdio>
#include <queue>
#include <cmath>
#include <vector>
#define mem(a,b) memset(a,b,sizeof(a))
using namespace std;
const int maxn = 1e3 + 10, INF = 0xfffffff, maxm = 100010;
typedef long long LL;
int n,m, cnt, s, t, k;
int head[maxn], d[maxn], vis[maxn], dis1[maxn], dis2[maxn], head1[maxn], cur[maxn];
int from[maxm], to[maxm];
int bz[55][55], way[55][55];

struct edge
{
    int u, v, c, next;
}Edge[maxm << 1];

void add_(int u, int v, int c)
{
    Edge[cnt].u = u;
    Edge[cnt].v = v;
    Edge[cnt].c = c;
    Edge[cnt].next = head1[u];
    head1[u] = cnt++;
}

void add_edge(int u, int v, int c)
{
    add_(u, v, c);
    add_(v, u, 0);
}

bool bfs()
{
    mem(d, 0);
    queue<int> Q;
    Q.push(s);
    d[s] = 1;
    while(!Q.empty())
    {
        int u = Q.front(); Q.pop();
        for(int i = head1[u]; i != -1; i = Edge[i].next)
        {
            edge e = Edge[i];
            if(!d[e.v] && e.c > 0)
            {
                d[e.v] = d[u] + 1;
                Q.push(e.v);
                if(e.v == t) return 1;
            }
        }
    }
    return d[t] != 0;
}

int dfs(int u, int cap)
{
    int ret = 0;
    if(u == t || cap == 0)
        return cap;
    for(int &i = cur[u]; i != -1; i = Edge[i].next)
    {
        edge e = Edge[i];
        if(d[e.v] == d[u] + 1 && e.c > 0)
        {
            int V = dfs(e.v, min(e.c, cap));
            Edge[i].c -= V;
            Edge[i ^ 1].c += V;
            ret += V;
            cap -= V;
            if(cap == 0) break;
        }
    }
    if(cap > 0) d[u] = -1;
    return ret;
}

int Dinic()
{
    int ans = 0;
    while(bfs())
    {
        memcpy(cur, head1, sizeof(head1));
        ans += dfs(s, INF);
    }
    return ans;
}

struct node
{
    int u, v, w, next;
}Node[maxn * 10];

void add(int u,int v,int w,int i)
{
    Node[i].u = u;
    Node[i].v = v;
    Node[i].w = w;
    Node[i].next = head[u];
    head[u] = i;
}
void spfa(int s)
{
    for(int i = 0; i < maxn; i++) d[i] = INF;
    queue<int> Q;
    d[s] = 0;
    mem(vis,0);
    Q.push(s);
    vis[s] = 1;
    while(!Q.empty())
    {
        int u = Q.front();Q.pop();
        vis[u] = 0;
        for(int i=head[u]; i!=-1; i=Node[i].next)
        {
            node e = Node[i];
            if(d[e.v] > d[u] + e.w)
            {
                d[e.v] = d[u] + e.w;
                if(!vis[e.v])
                {
                    Q.push(e.v);
                    vis[e.v] = 1;
                }
            }
        }

    }
}

int main()
{
    int T,A,B;
    while(~scanf("%d%d%d", &n, &m, &k))
    {
        if(n==0&&m==0&&k==0)
        break;
        mem(way, 0);
        mem(bz, 0);
        mem(Node,0);
        mem(head,-1);
        mem(head1, -1);
        cnt = 0;
        for(int i=0; i<m; i++)
        {
            scanf("%d%d",&from[i],&to[i]);
            if(!bz[from[i]][to[i]])
            {
                add(from[i],to[i],1,i), bz[from[i]][to[i]] = 1;
                way[from[i]][to[i]] = 1;
            }
        }
        s = 1, t = n;
        spfa(s);
        mem(Node,0);
        for(int i=1; i<=n; i++)
            dis1[i] = d[i];
        mem(head,-1);
        for(int i=0; i<m; i++)
            add(to[i],from[i],1,i);
        spfa(t);
        for(int i=1; i<=n; i++)
            dis2[i] = d[i];
        mem(bz, 0);
        s = 1 + n, t = n;
        for(int i = 1; i <= n; i++)
        {
            for(int j = 1; j <= n; j++)
            {
                if(way[i][j] && dis1[i] + dis2[j] + 1 <= k)
                    add_edge(i, j, INF);
            }
            add_edge(i, i + n, 1);
        }

        printf("%d
",Dinic());
    }
    return 0;
}