Submission #770860

#	Time	Username	Problem	Language	Result	Execution time	Memory
770860		PurpleCrayon	Building 4 (JOI20_building4)	C++17	100 / 100	205 ms	45348 KiB

building4

#include <bits/stdc++.h>
using namespace std;

#define sz(v) int(v.size())
#define ar array
typedef long long ll;
const int N = 1e6+10, MOD = 1e9+7;

// some are switchable, but only if the next thing switches (or if it's the tail end)
// you can just start from the end and swithc stuff while you need to

int n, a[N][2];
ar<int, 2> dp[N][2];

void add_self(ar<int, 2>& x, ar<int, 2> y, int delta) {
    y[0] += delta;
    y[1] += delta;
    x[0] = min(x[0], y[0]);
    x[1] = max(x[1], y[1]);
}

void solve() {
    cin >> n;
    for (int j = 0; j < 2; j++) {
        for (int i = 0; i < 2 * n; i++) cin >> a[i][j];
    }

    dp[0][0] = {-1, -1};
    dp[0][1] = {1, 1};
    for (int i = 1; i < 2 * n; i++) {
        // 1 if you take b, -1 if you take a
        dp[i][0] = {MOD, -MOD};
        dp[i][1] = {MOD, -MOD};

        for (int x = 0; x < 2; x++) {
            for (int y = 0; y < 2; y++) {
                if (a[i-1][x] <= a[i][y]) {
                    add_self(dp[i][y], dp[i-1][x], (y ? +1 : -1));
                }
            }
        }
    }

    int last = -1;
    for (int x : {0, 1}) if (dp[2*n-1][x][0] <= 0 && 0 <= dp[2*n-1][x][1]) last = x;
    if (last == -1) {
        cout << -1 << '\n';
        return;
    }


    string ans(2 * n, '?');
    int start = 0;
    for (int i = 2*n-1; i >= 0; i--) {
        ans[i] = last + 'A';
        if (!i) break;
        start -= (last ? +1 : -1);

        for (int x : {0, 1}) {
            if (a[i-1][x] <= a[i][last] && dp[i-1][x][0] <= start && start <= dp[i-1][x][1]) {
                last = x;
                break;
            }
        }
    }

    cout << ans << '\n';
}

int main() {
    ios::sync_with_stdio(false); cin.tie(0);
    int T = 1;
    // cin >> T;
    while (T--) solve();
}

• Subtask 1: 11 / 11
• Subtask 2: 89 / 89