bzoj 3175 [Tjoi2013]攻击装置

先黑白染色 再二分图最大独立集

#include<bits/stdc++.h>
using namespace std;
const int MAXN = 40050;
const int MAXM = 1000005;
const int INF = 1000000050;
int Head[MAXN], cur[MAXN], lev[MAXN], to[MAXM << 1], nxt[MAXM << 1], f[MAXM << 1], ed = 1, S, T;
inline void addedge(int u, int v, int cap)
{
    to[++ed] = v;
    nxt[ed] = Head[u];
    Head[u] = ed;
    f[ed] = cap;
    to[++ed] = u;
    nxt[ed] = Head[v];
    Head[v] = ed;
    f[ed] = 0;
    return;
}
inline bool BFS()
{
    int u;
    memset(lev, -1, sizeof(lev));
    queue<int>q;
    lev[S] = 0;
    q.push(S);
    while (q.size()) {
        u = q.front();
        q.pop();
        for (int i = Head[u]; i; i = nxt[i])
            if (f[i] && lev[to[i]] == -1) {
                lev[to[i]] = lev[u] + 1;
                q.push(to[i]);
                /*
                if (to[i] == T)
                {
                        return 1;
                }
                magic one way optimize
                */
            }
    }
    memcpy(cur, Head, sizeof Head);
    return lev[T] != -1;
}
inline int DFS(int u, int maxf)
{
    if (u == T || !maxf) {
        return maxf;
    }
    int cnt = 0;
    for (int &i = cur[u], tem; i; i = nxt[i])
        if (f[i] && lev[to[i]] == lev[u] + 1) {
            tem = DFS(to[i], min(maxf, f[i]));
            maxf -= tem;
            f[i] -= tem;
            f[i ^ 1] += tem;
            cnt += tem;
            if (!maxf) {
                break;
            }
        }
    if (!cnt) {
        lev[u] = -1;
    }
    return cnt;
}
int Dinic()
{
    int ans = 0;
    while (BFS()) {
        ans += DFS(S, 2147483647);
    }
    return ans;
}
void init(int SS, int TT)
{
    memset(Head, 0, sizeof(Head));
    ed = 1;
    S = SS;
    T = TT;
    return;
}
char ff[205][205];
pair<int, int> xy[400005];
int aim[205][205];
int cnt = 0;
const int dir[4][2] {{2, 1}, { -2, 1}, { -1, 2}, {1, 2}};
int main()
{
    int n;
    scanf("%d", &n);
    for (int i = 1; i <= n; i++) {
        scanf("%s", ff[i] + 1);
    }
    S = 0, T = 40001;
    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= n; j++) {
            if (ff[i][j] == '0') {
                xy[++cnt] = make_pair(i, j);
                aim[i][j] = cnt;
                if ((i + j) & 1) {
                    addedge(cnt, T, 1);
                } else {
                    addedge(S, cnt, 1);
                }
            }
        }
    }
    //cout<<cnt<<endl;
    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= n; j++) {
            if (ff[i][j] == '0')
                for (int k = 0; k <= 3; k++) {
                    int dx, dy;
                    dx = i + dir[k][0];
                    dy = j + dir[k][1];
                    if (dx >= 1 && dx <= n && dy >= 1 && dy <= n) {
                        if (ff[dx][dy] == '0') {
                            if ((i + j) & 1) {
                                addedge(aim[dx][dy], aim[i][j], 1);
                            } else {
                                addedge(aim[i][j], aim[dx][dy], 1);
                            }
                        }
                    }
                }
        }
    }
    int ans = cnt - Dinic();
    cout << ans << endl;
    return 0;
}