POJ 2135 Farm Tour 费用流入门题

 

#include<stdio.h>
#include<string.h>
#include<algorithm>
#include<queue>
using namespace std;
#define maxn 1010
#define maxm 10003
#define inf 1000000000

int min(int a, int b)
{
    return a < b ? a : b;
}

struct E
{
    int u, v, c, w, next;
}edge[maxm<<3];

int head[maxn], tot;
int n, m;
int S, T;

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

void add(int s, int t, int c, int w)
{
    edge[tot].u = s;
    edge[tot].v = t;
    edge[tot].c = c;
    edge[tot].w = w;
    edge[tot].next = head[s];
    head[s] = tot++;

    edge[tot].u = t;
    edge[tot].v = s;
    edge[tot].c = 0;
    edge[tot].w = -w;
    edge[tot].next = head[t];
    head[t] = tot++;
}

bool vis[maxn];
int pre[maxn];
int dis[maxn];
bool spfa(int s, int t, int n)
{
    int i, u, v;
    queue<int> q;
    for(i = 0; i <= n; i++)
        vis[i] = 0, pre[i] = -1, dis[i] = inf;
    vis[s] = 1; dis[s] = 0;
    q.push(s);
    while(!q.empty())
    {
        u = q.front(); q.pop(); vis[u] = 0;
        for(i = head[u]; i != -1; i = edge[i].next)
        {
            v = edge[i].v;
            if(edge[i].c && dis[v] > dis[u] + edge[i].w)
            {
                dis[v] = dis[u] + edge[i].w;
                pre[v] = i;
                if(!vis[v])
                {
                    q.push(v);
                    vis[v] = 1;
                }
            }
        }
    }
    if(dis[t] == inf) return 0;
    return 1;
}

int mcmf(int s, int t, int n)
{
    int maxf = 0;
    int i, aug, minw = 0;
    while(spfa(s, t, n))
    {
        aug = inf;
        for(i = pre[t]; i != -1; i = pre[edge[i].u])
            aug = min(aug, edge[i].c);
        maxf += aug;
        for(i = pre[t]; i != -1; i = pre[edge[i].u])
        {
            edge[i].c -= aug;
            edge[i^1].c += aug;
        }
        minw += dis[t] * aug;
    }
    return minw;
}

int main()
{
    int i, j;
    int x, y, z;
    while( ~scanf("%d%d", &n, &m))
    {
        init();
        S = 0; T = n+1;
        while(m--)
        {
            scanf("%d%d%d", &x, &y, &z);
            add(x, y, 1, z);
            add(y, x, 1, z);
        }
        add(S, 1, 2, 0);
        add(n, T, 2, 0);
        printf("%d\n", mcmf(S, T, T+1));
    }
    return 0;
}