D - Going Home POJ - 2195 网络流

Posted 2021-11-25 echozqn

On a grid map there are n little men and n houses. In each unit time, every little man can move one unit step, either horizontally, or vertically, to an adjacent point. For each little man, you need to pay a $1 travel fee for every step he moves, until he enters a house. The task is complicated with the restriction that each house can accommodate only one little man. 

Your task is to compute the minimum amount of money you need to pay in order to send these n little men into those n different houses. The input is a map of the scenario, a ‘.‘ means an empty space, an ‘H‘ represents a house on that point, and am ‘m‘ indicates there is a little man on that point. 

You can think of each point on the grid map as a quite large square, so it can hold n little men at the same time; also, it is okay if a little man steps on a grid with a house without entering that house.

Input

There are one or more test cases in the input. Each case starts with a line giving two integers N and M, where N is the number of rows of the map, and M is the number of columns. The rest of the input will be N lines describing the map. You may assume both N and M are between 2 and 100, inclusive. There will be the same number of ‘H‘s and ‘m‘s on the map; and there will be at most 100 houses. Input will terminate with 0 0 for N and M.

Output

For each test case, output one line with the single integer, which is the minimum amount, in dollars, you need to pay.

Sample Input

2 2
.m
H.
5 5
HH..m
.....
.....
.....
mm..H
7 8
...H....
...H....
...H....
mmmHmmmm
...H....
...H....
...H....
0 0

Sample Output

2
10
28



这个题目难在建图，就是把每一个人的位置，和每一个房子连起来，容量为1，费用为两个之间的距离。
然后就跑一个最小费用最大流就可以了。

#include <cstdio>
#include <cstdlib>
#include <queue>
#include <vector>
#include <iostream>
#include <algorithm>
#include <map>
#include <cstring>
#include <cmath>
#include <string>
#define inf 0x3f3f3f3f
using namespace std;
typedef long long ll;
const int INF = 0x3f3f3f3f;
const int maxn = 1000 + 10;
struct edge
{
    int u, v, c, f, cost;
    edge(int u, int v, int c, int f, int cost) :u(u), v(v), c(c), f(f), cost(cost) {}
};
vector<edge>e;
vector<int>G[maxn];
int a[maxn];//找增广路每个点的水流量
int p[maxn];//每次找增广路反向记录路径
int d[maxn];//SPFA算法的最短路
int inq[maxn];//SPFA算法是否在队列中
int s, t;
void init()
{
    for (int i = 0; i <= maxn; i++)G[i].clear();
    e.clear();
}
void add(int u, int v, int c, int cost)
{
    e.push_back(edge(u, v, c, 0, cost));
    e.push_back(edge(v, u, 0, 0, -cost));
    int m = e.size();
    G[u].push_back(m - 2);
    G[v].push_back(m - 1);
}
bool bellman(int s, int t, int& flow, long long & cost)
{
    memset(d, inf, sizeof(d));
    memset(inq, 0, sizeof(inq));
    d[s] = 0; inq[s] = 1;//源点s的距离设为0，标记入队
    p[s] = 0; a[s] = INF;//源点流量为INF（和之前的最大流算法是一样的）

    queue<int>q;//Bellman算法和增广路算法同步进行，沿着最短路拓展增广路，得出的解一定是最小费用最大流
    q.push(s);
    while (!q.empty())
    {
        int u = q.front();
        q.pop();
        inq[u] = 0;//入队列标记删除
        for (int i = 0; i < G[u].size(); i++)
        {
            edge & now = e[G[u][i]];
            int v = now.v;
            if (now.c > now.f && d[v] > d[u] + now.cost)
                //now.c > now.f表示这条路还未流满（和最大流一样）
                //d[v] > d[u] + e.cost Bellman 算法中边的松弛
            {
                d[v] = d[u] + now.cost;//Bellman 算法边的松弛
                p[v] = G[u][i];//反向记录边的编号
                a[v] = min(a[u], now.c - now.f);//到达v点的水量取决于边剩余的容量和u点的水量
                if (!inq[v]) { q.push(v); inq[v] = 1; }//Bellman 算法入队
            }
        }
    }
    if (d[t] == INF)return false;//找不到增广路
    flow += a[t];//最大流的值，此函数引用flow这个值，最后可以直接求出flow
    cost += (long long)d[t] * (long long)a[t];//距离乘上到达汇点的流量就是费用
    for (int u = t; u != s; u = e[p[u]].u)//逆向存边
    {
        e[p[u]].f += a[t];//正向边加上流量
        e[p[u] ^ 1].f -= a[t];//反向边减去流量 （和增广路算法一样）
    }
    return true;
}
int MincostMaxflow(int s, int t, long long & cost)
{
    cost = 0;
    int flow = 0;
    while (bellman(s, t, flow, cost));//由于Bellman函数用的是引用，所以只要一直调用就可以求出flow和cost
    return flow;//返回最大流，cost引用可以直接返回最小费用
}
struct node
{
    int x, y;
    node(int x=0,int y=0):x(x),y(y){}
};
node peo[110], house[110];
char mp[110][110];
int main()
{
    int n, m;
    while(cin>>n>>m)
    {
        init();
        int cas = 0, tot = 0;
        if (n == 0 && m == 0) break;
        for (int i = 1; i <= n; i++)
        {
            cin >> mp[i] + 1;
            for(int j=1;j<=m;j++)
            {
                if (mp[i][j] == ‘m‘) peo[++cas] = node(i, j);
                if (mp[i][j] == ‘H‘) house[++tot] = node(i, j);
            }
        }
        s = 0, t = cas + tot + 1;
        for (int i = 1; i <= cas; i++) add(s, i, 1, 0);
        for (int i = 1; i <= tot; i++) add(cas + i, t, 1, 0);
        for(int i=1;i<=cas;i++)
        {
            for(int j=1;j<=tot;j++)
            {
                int cost = abs(peo[i].x - house[j].x) + abs(peo[i].y - house[j].y);
                add(i, j + cas, 1, cost);
            }
        }
        ll cost = 0;
        int ans = MincostMaxflow(s, t, cost);
        printf("%lld\n", cost);
    }
    return 0;
}