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136
LibreOJ/6226/6226.cpp
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136
LibreOJ/6226/6226.cpp
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#include <iostream>
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#include <algorithm>
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#include <queue>
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using std::cin;
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using std::cout;
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const char endl = '\n';
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const int N = 205,
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M = N * N;
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const int INF = 0x3f3f3f3f;
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const int d[][2]{{1, 2}, {1, -2}, {-1, 2}, {-1, -2}, {2, 1}, {2, -1}, {-2, 1}, {-2, -1}};
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int n, m, s, t;
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int idx, head[M << 1], ver[M << 4], edge[M << 4], next[M << 4];
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int dis[M << 1], cur[M << 1];
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bool g[N][N];
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void add(int u, int v, int w) {
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next[idx] = head[u];
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ver[idx] = v;
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edge[idx] = w;
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head[u] = idx++;
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}
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int id(int x, int y) {
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return x * n + y;
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}
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bool bfs() {
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std::fill_n(dis, M << 1, 0);
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std::queue<int> q;
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q.emplace(s);
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dis[s] = 1;
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cur[s] = head[s];
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while (!q.empty()) {
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int u = q.front();
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q.pop();
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for (int i = head[u]; ~i; i = next[i]) {
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int v = ver[i],
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w = edge[i];
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if (w && !dis[v]) {
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dis[v] = dis[u] + 1;
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cur[v] = head[v];
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if (v == t) return true;
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q.emplace(v);
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}
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}
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}
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return false;
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}
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int dinic(int u, int limit) {
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if (u == t) return limit;
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int flow = 0;
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for (int i = cur[u]; ~i && flow < limit; i = next[i]) {
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cur[u] = i;
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int v = ver[i],
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w = edge[i];
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if (w && dis[v] == dis[u] + 1) {
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int k = dinic(v, std::min(w, limit - flow));
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if (!k) dis[v] = 0;
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edge[i] -= k;
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edge[i ^ 1] += k;
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flow += k;
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}
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}
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return flow;
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}
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int main() {
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std::ios::sync_with_stdio(false);
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cin.tie(nullptr);
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std::fill_n(head, M << 1, -1);
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cin >> n >> m;
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for (int i = 1, x, y; i <= m; i++) {
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cin >> x >> y;
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g[x][y] = true;
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}
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s = 0, t = id(n, n) + 1;
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for (int i = 1; i <= n; i++) {
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for (int j = 1; j <= n; j++) {
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if (g[i][j]) continue;
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if ((i + j) % 2) {
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add(s, id(i, j), 1);
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add(id(i, j), s, 0);
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for (auto d : d) {
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int x = i + d[0],
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y = j + d[1];
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if (x < 1 || x > n || y < 1 || y > n || g[x][y]) continue;
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add(id(i, j), id(x, y), INF);
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add(id(x, y), id(i, j), 0);
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}
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} else {
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add(id(i, j), t, 1);
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add(t, id(i, j), 0);
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}
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}
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}
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int ans = 0, flow;
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while (bfs()) {
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while (flow = dinic(s, INF)) ans += flow;
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}
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std::cerr << ans << endl;
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cout << n * n - m - ans << endl;
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return 0;
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}
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