Codeforces Round #375 (Div. 2) D. Lakes in Berland DFS

D. Lakes in Berland

链接：

http://codeforces.com/problemset/problem/723/D

题意

给你一个n/*m的矩阵，然后你们有不少于k条湖泊，然后你需要使得一些湖泊变成陆地，使得湖泊的数量恰好等于k，问你至少填多少个水。

湖泊不与外界相邻。

题解：

直接dfs搜出每一条湖泊，然后放在优先队列里，从小到大去填满就好了。

代码：

#include<iostream>
#include<queue>
#include<vector>
#include<functional>
using namespace std;

typedef pair<int, int> P;
typedef pair<int, P> PP;
const int maxn = 55;
char map[maxn][maxn];
int vis[maxn][maxn];
int dx[4] = { 1,0,-1,0 };
int dy[4] = { 0,1,0,-1 };
int n, m, sum, fg;
priority_queue<PP, vector<PP>, greater<PP> > lake;

void dfs(P s)
{
    int x = s.first, y = s.second;
    sum++;
    vis[x][y] = 1;
    if (x == 1 || x == n || y == 1 || y == m) fg = 0;
    for (int i = 0; i < 4; i++)
    {
        int nx = x + dx[i];
        int ny = y + dy[i];
        if (nx >= 1 && nx <= n && ny >= 1 && ny <= m &&
            map[nx][ny] == '.' && !vis[nx][ny]) 
        dfs(P(nx, ny));
    }
}

void dfs2(P s)
{
    int x = s.first, y = s.second;
    map[x][y] = '*';
    for (int i = 0; i < 4; i++)
    {
        int nx = x + dx[i];
        int ny = y + dy[i];
        if (nx >= 1 && nx <= n && ny >= 1 && ny <= m &&
            map[nx][ny] == '.')
            dfs2(P(nx, ny));
    }
}

int main()
{
    int k;
    cin >> n >> m >> k;
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= m; j++)
            cin >> map[i][j];
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= m; j++)
            if (map[i][j] == '.' && vis[i][j] == 0) {
                fg = 1;
                sum = 0;
                dfs(P(i, j));
                if (fg) lake.push(PP(sum, P(i, j)));
            }
    int ans = 0;
    while (lake.size() != k) {
        ans += lake.top().first;
        dfs2(lake.top().second);
        lake.pop();
    }
    cout << ans << endl;
    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= m; j++)
            cout << map[i][j];
        cout << endl;
    }
    return 0;
}