HDU 2485 Destroying the bus stations

2015 ACM / ICPC 北京站 热身赛 C题

#include<cstdio>
#include<cstring>
#include<cmath>
#include<queue>
#include<vector>
#include<algorithm>
using namespace std;

const int INF=0x7FFFFFFF;
const int maxn=50+10;//点的数量
int n,m,k;
int u[4000+10],v[4000+10];
int dis1[maxn],dis2[maxn];
bool flag1[maxn],flag2[maxn];
vector<int>P[maxn];
vector<int>FP[maxn];
queue<int>Q1;
queue<int>Q2;

struct Edge
{
    int from, to, cap, flow;
    Edge(int u, int v, int c, int f) :from(u), to(v), cap(c), flow(f) {}
};
vector<Edge>edges;
vector<int>G[maxn*2];
bool vis[maxn*2];
int d[maxn*2];
int cur[maxn*2];
int s, t;

void read()
{
    for(int i=1;i<=m;i++)
    {
        scanf("%d%d",&u[i],&v[i]);
        P[u[i]].push_back(v[i]);
        FP[v[i]].push_back(u[i]);
    }
}

void init()
{
    for(int i=0;i<maxn;i++) P[i].clear();
    for(int i=0;i<maxn;i++) FP[i].clear();

    for(int i=1;i<=n;i++) dis1[i]=INF,dis2[i]=INF;

    for(int i = 0; i < 2*maxn; i++) G[i].clear();
    edges.clear();
}

void AddEdge(int from, int to, int cap)
{
    edges.push_back(Edge(from, to, cap, 0));
    edges.push_back(Edge(to, from, 0, 0));
    int w = edges.size();
    G[from].push_back(w - 2);
    G[to].push_back(w - 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 = 0; i<G[x].size(); i++)
        {
            Edge e = edges[G[x][i]];
            if (!vis[e.to] && e.cap>e.flow)
            {
                vis[e.to] = 1;
                d[e.to] = d[x] + 1;
                Q.push(e.to);
            }
        }
    }
    return vis[t];
}

int DFS(int x, int a)
{
    if (x == t || a == 0)
        return a;
    int flow = 0, f;
    for (int &i = cur[x]; i<G[x].size(); i++)
    {
        Edge e = edges[G[x][i]];
        if (d[x]+1 == d[e.to]&&(f=DFS(e.to,min(a,e.cap-e.flow)))>0)
        {
            edges[G[x][i]].flow+=f;
            edges[G[x][i] ^ 1].flow-=f;
            flow+=f;
            a-=f;
            if(a==0) break;
        }
    }
    if(!flow) d[x] = -1;
    return flow;
}

int dinic(int s, int t)
{
    int flow = 0;
    while (BFS())
    {
        memset(cur, 0, sizeof(cur));
        flow += DFS(s, INF);
    }
    return flow;
}

void spfa1(int x)
{
    while(!Q1.empty()) Q1.pop();
    memset(flag1,0,sizeof flag1);

    dis1[x]=0;
    flag1[x]=1;
    Q1.push(x);

    while(!Q1.empty())
    {
        int h=Q1.front(); flag1[h]=0; Q1.pop();
        for(int i=0;i<P[h].size();i++)
        {
            if(dis1[h]+1<dis1[P[h][i]])
            {
                dis1[P[h][i]]=dis1[h]+1;
                if(flag1[P[h][i]]==0)
                {
                    flag1[P[h][i]]=1;
                    Q1.push(P[h][i]);
                }
            }
        }
    }
}

void spfa2(int x)
{
    while(!Q2.empty()) Q2.pop();
    memset(flag2,0,sizeof flag2);

    dis2[x]=0;
    flag2[x]=1;
    Q2.push(x);

    while(!Q2.empty())
    {
        int h=Q2.front(); flag2[h]=0; Q2.pop();
        for(int i=0;i<FP[h].size();i++)
        {
            if(dis2[h]+1<dis2[FP[h][i]])
            {
                dis2[FP[h][i]]=dis2[h]+1;
                if(flag2[FP[h][i]]==0)
                {
                    flag2[FP[h][i]]=1;
                    Q2.push(FP[h][i]);
                }
            }
        }
    }
}

int main()
{
    while(~scanf("%d%d%d",&n,&m,&k))
    {
        if(!n&&!m&&!k) break;

        init();
        read();

        spfa1(1);
        spfa2(n);

        s=1;t=n+n;

        for(int i=1;i<=n;i++) AddEdge(i+n,i,1);

        for(int i=1;i<=m;i++)
            if(dis1[u[i]]+dis2[v[i]]+1<=k)
                AddEdge(u[i],v[i]+n,INF);

        int ans=dinic(s,t);
        printf("%d\n",ans);
    }
    return 0;
}