public class Day15 {

public static void main(String[] args) {

final int[] challengeInput = {14, 8, 16, 0, 1, 17};

play(challengeInput, 2020); // part 1: 240

play(challengeInput, 300_000_00); // part 2: 505

}

public static void play(final int[] startNumbers, final int endTurn) {

final int[] memory = new int[endTurn];

int turn = 0;

int lastSeen = 0;

for (int sn : startNumbers) {

lastSeen = memory[sn];

memory[sn] = ++turn;

}

int num = 0;

while (turn < endTurn) {

num = lastSeen == 0 ? 0 : turn - lastSeen;

lastSeen = memory[num];

memory[num] = ++turn;

}

System.out.println(num);

}

}