POJ 2226 最小点覆盖（经典建图）

  1 #include <cstdio>
  2 #include <cstring>
  3 #include <algorithm>
  4 #include <iostream>
  5 #include <vector>
  6 #include <queue>
  7 #include <cmath>
  8 #include <set>
  9 using namespace std;
 10 
 11 #define N 50
 12 
 13 int max(int x,int y){return x>y?x:y;}
 14 int min(int x,int y){return x<y?x:y;}
 15 int abs(int x,int y){return x<0?-x:x;}
 16 
 17 int n, m;
 18 vector<int>ve[N*N];
 19 int from[N*N];
 20 bool visited[N*N];
 21 char map[N][N];    
 22 int map1[N][N];
 23 int map2[N][N];
 24 
 25 int march(int u){
 26     int i, v;
 27     for(i=0;i<ve[u].size();i++){
 28         v=ve[u][i];
 29         if(!visited[v]){
 30             visited[v]=true;
 31             if(from[v]==-1||march(from[v])){
 32                 from[v]=u;
 33                 return 1;
 34             }
 35         }
 36     }
 37     return 0;
 38 }
 39 
 40 main()
 41 {
 42     int i, j, k;
 43     while(scanf("%d %d",&n,&m)==2){
 44         for(i=0;i<n;i++) scanf("%s",map[i]);
 45         memset(map1,-1,sizeof(map1));
 46         memset(map2,-1,sizeof(map2));
 47         int num=1;
 48         int maxh=0;
 49         //横着处理
 50         for(i=0;i<n;i++){
 51             for(j=0;j<m;j++){
 52                 if(map[i][j]==‘*‘){
 53                     if(j==0) map1[i][j]=num;
 54                     else{
 55                         if(map[i][j-1]==‘*‘) map1[i][j]=map1[i][j-1];
 56                         else map1[i][j]=num;
 57                     }
 58                 }
 59                 else {
 60                     if(j<m-1&&map[i][j+1]==‘*‘) num++;
 61                 }
 62             }
 63             num++;
 64             maxh=max(maxh,num);
 65         } 
 66         //竖着处理 
 67         num=1;
 68         for(j=0;j<m;j++){
 69             for(i=0;i<n;i++){
 70                 if(map[i][j]==‘*‘){
 71                     if(i==0) map2[i][j]=num;
 72                     else{
 73                         if(map[i-1][j]==‘*‘) map2[i][j]=map2[i-1][j];
 74                         else map2[i][j]=num;
 75                     }
 76                 }
 77                 else{
 78                     if(i<n-1&&map[i+1][j]==‘*‘) num++;
 79                 }
 80             }
 81             num++;
 82             maxh=max(maxh,num);
 83         }
 84         //建二分图 
 85         for(i=0;i<maxh;i++) ve[i].clear();
 86         for(i=0;i<n;i++){
 87             for(j=0;j<m;j++){
 88                 if(map1[i][j]!=-1&&map2[i][j]!=-1){
 89                     ve[map1[i][j]].push_back(map2[i][j]);
 90                 }
 91             }
 92         }
 93         //二分匹配 
 94         memset(from,-1,sizeof(from));
 95         num=0;
 96         for(i=0;i<maxh;i++){
 97             memset(visited,false,sizeof(visited));
 98             if(march(i)) num++;
 99         }
100         printf("%d\n",num);
101     }
102 }