Submission #684199

#	Time	Username	Problem	Language	Result	Execution time	Memory
684199		Tigerpants	Rack (eJOI19_rack)	C++17	40 / 100	2 ms	468 KiB

rack

#include <iostream>
#include <vector>
#include <set>
#include <algorithm>
#include <map>
#include <numeric>
#include <iomanip>

using namespace std;

typedef long long int ll;
typedef vector<ll> vll;
typedef vector<vll> vvll;

const ll mod = 1000000007;

vll po2;

ll pow2(ll x) {
    if (x >= po2.size()) {
        if (x & 1) {
            po2[x] = (2 * pow2(x - 1)) % mod;
        } else {
            po2[x] = (pow2(x / 2) * pow2(x / 2)) % mod;
        }
    }
    return po2[x];
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    ll n, k;
    cin >> n >> k;
    
    k -= 1;

    po2.push_back(1);

    // for a structure with n levels, on which hook should the k'th coat hang
    // consinder hanging coats one by one. for each rack, it alternates whether it should be hung to the left or right, always starting with the left
    // so we check the bitrepresentation of k 

    // 0000, 1000, 0100, 1100, 0010, 1010, 0110, 1110, 0001, 1001, 0101, 1101, 0011, 1011, 0111, 1111
    // so the answer is k reversed mod 2 ^ n... 

    ll answ = 0;
    for (int i = 0; i < 63; i++) {
        if ((k >> i) & 1) {
            answ += pow2(n - 1 - i);
            answ %= mod;
        }
    }

    cout << answ + 1 << endl;

    return 0;
}

Compilation message (stderr)

rack.cpp: In function 'll pow2(ll)':
rack.cpp:20:11: warning: comparison of integer expressions of different signedness: 'll' {aka 'long long int'} and 'std::vector<long long int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   20 |     if (x >= po2.size()) {
      |         ~~^~~~~~~~~~~~~

• Subtask 1: 20 / 20
• Subtask 2: 20 / 20
• Subtask 3: 0 / 60