Submission #807116

#	Time	Username	Problem	Language	Result	Execution time	Memory
807116		GrindMachine	Building 4 (JOI20_building4)	C++17	100 / 100	203 ms	68900 KiB

building4

// Om Namah Shivaya

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

template<typename T> using Tree = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
typedef long long int ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

#define fastio ios_base::sync_with_stdio(false); cin.tie(NULL)
#define pb push_back
#define endl '\n'
#define sz(a) a.size()
#define setbits(x) __builtin_popcountll(x)
#define ff first
#define ss second
#define conts continue
#define ceil2(x, y) ((x + y - 1) / (y))
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define yes cout << "Yes" << endl
#define no cout << "No" << endl

#define rep(i, n) for(int i = 0; i < n; ++i)
#define rep1(i, n) for(int i = 1; i <= n; ++i)
#define rev(i, s, e) for(int i = s; i >= e; --i)
#define trav(i, a) for(auto &i : a)

template<typename T>
void amin(T &a, T b) {
    a = min(a, b);
}

template<typename T>
void amax(T &a, T b) {
    a = max(a, b);
}

#ifdef LOCAL
#include "debug.h"
#else
#define debug(x) 42
#endif

/*

refs:
https://codeforces.com/blog/entry/74871?#comment-590839
proof for lemma and a nicer way to think about the problem => https://codeforces.com/blog/entry/74871?#comment-590887

*/

const int MOD = 1e9 + 7;
const int N = 1e5 + 5;
const int inf1 = int(1e9) + 5;
const ll inf2 = ll(1e18) + 5;

void solve(int test_case)
{
    ll n; cin >> n;
    vector<ll> a(2*n+5), b(2*n+5);
    rep1(i,2*n) cin >> a[i];
    rep1(i,2*n) cin >> b[i];

    pll dp[2*n+5][2];
    rep(i,2*n+5) rep(j,2) dp[i][j] = {inf2,-inf2};
    dp[0][0] = {0,0};

    rep1(i,2*n){
        rep(x,2){
            rep(y,2){
                ll v1 = a[i], v2 = a[i-1];
                if(x) v1 = b[i];
                if(y) v2 = b[i-1];

                if(v1 >= v2){
                    amin(dp[i][x].ff,dp[i-1][y].ff+(x^1));
                    amax(dp[i][x].ss,dp[i-1][y].ss+(x^1));
                }
            }
        }
    }

    ll x = -1;
    rep(j,2){
        if(dp[2*n][j].ff <= n and dp[2*n][j].ss >= n){
            x = j;
        }
    }

    if(x == -1){
        cout << -1 << endl;
        return;
    }

    string ans = "";
    ll want = n;

    rev(i,2*n,1){
        if(!x) ans.pb('A'), want--;
        else ans.pb('B');

        bool ok = false;

        rep(y,2){
            ll v1 = a[i], v2 = a[i-1];
            if(x) v1 = b[i];
            if(y) v2 = b[i-1];

            if(v1 >= v2){
                if(dp[i-1][y].ff <= want and dp[i-1][y].ss >= want){
                    x = y;
                    ok = true;
                    break;
                }
            }
        }

        assert(ok);
    }

    assert(want == 0);

    reverse(all(ans));
    cout << ans << endl;
}

int main()
{
    fastio;

    int t = 1;
    // cin >> t;

    rep1(i, t) {
        solve(i);
    }

    return 0;
}

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