codevs2171 棋盘覆盖

#include <cstdio>
#include <vector>
#include <cstring>
#include <iostream>
#include <algorithm>

using namespace std;
typedef vector<int>::iterator iterator_t;
int n, m, ans, pipei[10010];
int dx[] = { 0, -1, 0, 1 }, dy[] = { -1, 0, 1, 0 };
int flag[110][110],vis[10010];
int tot = 1, to[110010], nextt[100010], head[100010];
vector <int>e[10010];

void add(int x, int y)
{
    to[tot] = y;
    nextt[tot] = head[x];
    head[x] = tot++;
}

bool judge(int x, int y)
{
    if (x < 1 || x > n || y < 1 || y > n || flag[x][y])
        return false;
    return true;
}

bool dfs(int u)
{
    for (int i = head[u]; i;i = nextt[i])
    {
        int v = to[i];
        if (!vis[v])
        {
            vis[v] = 1;
            if ((pipei[v] == -1)|| dfs(pipei[v]))
            {
                pipei[v] = u;
                pipei[u] = v;
                return true;
            }
        }
    }
    return false;
}

int main()
{
    scanf("%d%d", &n, &m);
    for (int i = 1; i <= m; i++)
    {
        int x, y;
        scanf("%d%d", &x, &y);
        flag[x][y] = 1;
    }
    for (int i = 1; i <= n; i++)
    {
        for (int j = 1; j <= n; j++)
        {
            if (flag[i][j])
                continue;
                for (int k = 0; k < 4; k++)
                {
                    int nx = i + dx[k], ny = j + dy[k];
                    if (judge(nx,ny))
                    {
                        int t1 = (i - 1) * n + j , t2 = (nx - 1) * n + ny;
                        add(t1, t2);
                    }
                }
            
        }
    }
    memset(pipei, -1, sizeof(pipei));
    for (int i = 0; i < n * n; i++)
    {
        if (pipei[i] == -1)
        {
            memset(vis, 0, sizeof(vis));
            if (dfs(i))
            ans++;
        }
    }
    printf("%d\n", ans);

    return 0;
}