Going Home POJ - 2195（最小费用流）

Going Home POJ - 2195（最小费用流）

题目链接
题目大意：给定一个N*M的地图，地图上有若干个man和house，且man与house的数量一致。man每移动一格需花费1（即单位费用=单位距离），一间house只能入住一个man。现在要求所有的man都入住house，求最小费用。
样例：
2 2
.m
H.
5 5
HH..m
…..
…..
…..
mm..H
7 8
…H….
…H….
…H….
mmmHmmmm
…H….
…H….
…H….
0 0
输出：
2
10
28

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

const int maxn = 10012+ 10;
const int INF = 0x3f3f3f3f;

struct node {
   int u, to, w, cost, next;
} Edge[maxn << 2];

struct Point {
   int x, y, flag;
};

vector<Point> p;

int Head[maxn], cnt, dist[maxn], flow[maxn], pre[maxn];
char str[maxn];
bool vis[maxn];

int GetDistance(Point a, Point b) {
  return abs(a.x - b.x) + abs(a.y - b.y);
}

void add(int u, int v, int w) {
   Edge[cnt] = (node) {u, v, 1, w, Head[u]};
   Head[u] = cnt++;
   Edge[cnt] = (node) {v, u, 0, -w, Head[v]};
   Head[v] = cnt++;
}

bool spfa(int s, int e) {
   queue<int> q;
   q.push(s);
   memset(dist, INF, sizeof(dist));
   memset(vis, false, sizeof(vis));
   dist[s] = 0;
   vis[s] = true;
   flow[s] = INF;

   while(!q.empty()) {
     int now = q.front();
     q.pop();
     vis[now] = false;
     for(int i = Head[now]; ~i; i = Edge[i].next) {
        int to = Edge[i].to, w = Edge[i].w, cost = Edge[i].cost;
        if(w > 0 && dist[to] > dist[now] + cost) {
             dist[to] = dist[now] + cost;
             flow[to] = min(flow[now], w);
             if(!vis[to]) {
                q.push(to);
                vis[to] = true;
             }

             pre[to] = i;
        }
     }
   }

   return dist[e] != INF;
}

int min_cost_flow(int s, int e) {
   int res = 0;
   while(spfa(s, e)) {
       int u = e;
       res += dist[e] * flow[e];
       while(u != s) {
          int now = pre[u];
          Edge[now].w -= flow[e];
          Edge[now ^ 1].w += flow[e];
          u = Edge[now].u;
       }
   }
   return res;
}

int main()
{
    int n, m;
    while(~scanf("%d %d", &n, &m) && (n + m)) {
        p.clear();
        for(int i = 0; i < n; i++) {
            scanf("%s", str);
            for(int j = 0; j < m; j++) {
                if(str[j] == 'm') p.push_back((Point) {i, j, true});
                if(str[j] == 'H') p.push_back((Point) {i, j, false});
            }
        }

        cnt = 0;
        memset(Head, -1, sizeof(Head));

        int s = p.size(), e = s + 1;
        for(int i = 0; i < s; i++) {
            if(p[i].flag) {
                add(s, i, 0);
                for(int j = 0; j < s; j++) {
                    if(!p[j].flag) {
                        add(i, j, GetDistance(p[i], p[j]));
                    }
                }
            }
            else add(i, e, 0);
        }

        int v = min_cost_flow(s, e);
        printf("%d\n", v);
    }
}