UVA - 1658 Admiral

3. C - Admiral

思路：

拆点+最小费用最大流

解题代码：

#include <cstdio>

#include <cstring>

#include <queue>

#include <vector>

using namespace std;

#define rep(i,a,n) for(int i = a; i < n; i++)

#define repe(i,a,n) for(int i = a; i <= n; i++)

#define clc(a,b) memset(a,b,sizeof(a))

#define MAXN 2010

#define INF 0x3f3f3f3f

typedef long long LL;

struct MCMF{

    int n,m;

    struct Edge{

        int from, to, cap, flow, cost;

        Edge(int a, int b, int c, int d, int e){

            from = a, to = b, cap = c, flow = d, cost = e;

        }

    };

    vector<Edge> edge;

    vector<int> g[MAXN];

    bool inq[MAXN];

    int d[MAXN]/*spfa*/, p[MAXN]/*上一条弧*/, a[MAXN]/*可改进量*/;

    void init(int n){

        this->n = n;

        repe(i,1,n) g[i].clear();

        edge.clear();

    }

    void add_edge(int from, int to, int cap, int cost)

    {

        edge.push_back(Edge(from,to,cap,0,cost));

        edge.push_back(Edge(to,from,0,0,-cost));

        m = edge.size();

        g[from].push_back(m-2);

        g[to].push_back(m-1);

    }

    bool spfa(int s, int t, int& flow, LL& cost)

    {

        clc(d,0x3f);

        clc(inq,0);

        d[s] = 0, inq[s] = true, p[s] = 0, a[s] = INF;

        queue<int> q;

        q.push(s);

        while(!q.empty())

        {

            int u = q.front();q.pop();

            inq[u] = false;

            int sz = g[u].size();

            rep(i,0,sz)

            {

                Edge& e = edge[g[u][i]];

                if(e.cap > e.flow && d[e.to] > d[u]+e.cost)

                {

                    d[e.to] = d[u]+e.cost;

                    p[e.to] = g[u][i];

                    a[e.to] = min(a[u], e.cap-e.flow);

                    if(!inq[e.to]) q.push(e.to), inq[e.to] = true;

                }

            }

        }

        if(INF == d[t]) return false;

        flow += a[t];

        cost += (LL)d[t]*(LL)a[t];

        for(int u = t; u != s; u = edge[p[u]].from)

        {

            edge[p[u]].flow += a[t];

            edge[p[u]^1].flow -= a[t];

        }

        return true;

    }

    //需要保证初始网络没有负圈，返回最大流量，cost才是最小花费

    int mincostmaxflow(int s, int t, LL & cost)

    {

        int flow = 0;

        cost = 0;

        while(spfa(s,t,flow,cost));

        return flow;

    }

}mcmf;

int main()

{

#ifdef SHY

    freopen("e:\\1.txt","r",stdin);

#endif

    int n,m;

    while(~scanf("%d %d%*c", &n, &m))

    {

        int a,b,c;

        mcmf.init(n<<1);

        rep(i,2,n) mcmf.add_edge(i,i+n,1,0);

        mcmf.add_edge(1,n+1,2,0);

        mcmf.add_edge(n,n<<1,2,0);

        rep(i,0,m)

        {

            scanf("%d %d %d%*c", &a, &b, &c);

            mcmf.add_edge(a+n,b,1,c);

        }

        LL ans;

        mcmf.mincostmaxflow(1,n<<1,ans);

        printf("%lld\n", ans);

    }

    return 0;

}