Codeforces 723D. Lakes in Berland

解题思路：

　　1.dfs所有的水，顺便计数大小并判断是不是湖。

　　2.如果是湖，将大小和坐标存下来。

　　3.对湖按大小从小到大排序。

　　4.dfs前（湖的数量-k）个湖，用*填充这些湖。

代码：

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;struct lake{int x;int y;int size;bool islake;
};
vector <lake> l;char a[55][55];
bool used[55][55];
int n,m,k;
int dir[4][2] = {1,0,-1,0,0,1,0,-1};bool cmp(lake p,lake q){return p.size < q.size;
}void dfs(int x,int y,lake &t){
//    cout << x << " " << y << " l" << endl;if(x == 1 or y == 1 or x == n or y == m) t.islake = false;t.size++;used[x][y] = true;for(int i = 0;i < 4; ++i){int conx = x+dir[i][0];int cony = y+dir[i][1];if(!used[conx][cony] and a[conx][cony] == '.'){dfs(conx, cony, t);}}
}void dfsfill(int x,int y){a[x][y] = '*';for(int i = 0;i < 4; ++i){int conx = x+dir[i][0];int cony = y+dir[i][1];if(a[conx][cony] == '.'){dfsfill(conx, cony);}}
}int main(){ios::sync_with_stdio(false);cin >> n >> m >> k;for(int i = 1;i <= n; ++i) cin >> a[i]+1;     for(int i = 1;i <= n; ++i){for(int j = 1;j <= m; ++j){if(!used[i][j] and a[i][j] == '.'){lake t;t.x = i;t.y = j;t.size = 0;t.islake = true;dfs(i, j, t);if(t.islake){
//                    cout << t.x << " " << t.y <<" " << t.size << endl;l.push_back(t);}}}}sort(l.begin(),l.end(),cmp);int s = l.size() - k;int ans = 0;for(int i = 0;i < s; ++i){ans += l[i].size;dfsfill(l[i].x, l[i].y);}cout << ans << endl;for(int i = 1;i <= n; ++i) cout << a[i]+1 << endl;return 0;
}

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