「SNOI2019」通信 分治建图

 根据题意 每个点可以直接与S,T相连 也可以和前面的哨站相连 暴力建边的话 有n²条边

要用分治优化建边:

类似于归并排序 先对每一层分为左半边与右半边 对每一半都拿出来先排序去重后 直接排成一条链建边

if (l == r) {
                return ;
        }
        int mid = (l + r) >> 1;
        solve(l, mid), solve(mid + 1, r);
        int cnt = 0;
        for (int i = l; i <= r; i++) {
                aa[++cnt] = a[i];
        }
        sort(aa + 1, aa + 1 + cnt);
        cnt = unique(aa + 1, aa + cnt + 1) - aa - 1;
        int now = MCMF::MAXP;
        for (int i = 1; i < cnt; i++) {
                MCMF::addedge(now + i, now + i + 1, inf, aa[i + 1] - aa[i]);
                MCMF::addedge(now + i + 1, now + i, inf, aa[i + 1] - aa[i]);
        }

然后对于区间内的每个数 前半边的出后半边的入

        for (int i = l; i <= r; i++) {
                int aim = lower_bound(aa + 1, aa + cnt + 1, a[i]) - aa;
                if (i <= mid) {
                        MCMF::addedge(now + aim, i + n, 1, 0);
                } else {
                        MCMF::addedge(i, now + aim, 1, 0);
                }
        }

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long long JQK;
const int inf = INT_MAX / 2;int n, a[1005], aa[1005];
void solve(int l, int r) {
        if (l == r) {
                return ;
        }
        int mid = (l + r) >> 1;
        solve(l, mid), solve(mid + 1, r);
        int cnt = 0;
        for (int i = l; i <= r; i++) {
                aa[++cnt] = a[i];
        }
        sort(aa + 1, aa + 1 + cnt);
        cnt = unique(aa + 1, aa + cnt + 1) - aa - 1;
        int now = MCMF::MAXP;
        for (int i = 1; i < cnt; i++) {
                MCMF::addedge(now + i, now + i + 1, inf, aa[i + 1] - aa[i]);
                MCMF::addedge(now + i + 1, now + i, inf, aa[i + 1] - aa[i]);
        }
        for (int i = l; i <= r; i++) {
                int aim = lower_bound(aa + 1, aa + cnt + 1, a[i]) - aa;
                if (i <= mid) {
                        MCMF::addedge(now + aim, i + n, 1, 0);
                } else {
                        MCMF::addedge(i, now + aim, 1, 0);
                }
        }
        MCMF::MAXP += cnt;
}
int main() {
        int x;
        int s, t;
        ll W;
        RR(n), RR(x);
        W = x;
        MCMF::MAXP = 2 * n + 3, s = 2 * n + 1, t = 2 * n + 2;
        for (int i = 1; i <= n; i++) {
                RR(a[i]);
                MCMF::addedge(s, i, 1, 0), MCMF::addedge(i, t, 1, W), MCMF::addedge(i + n, t, 1, 0);
        }
        solve(1, n);
        MCMF::init(s, t);
        cout << MCMF::MCMF() << "
";
        return 0;
}