g0901_1000.s0957_prison_cells_after_n_days.Solution Maven / Gradle / Ivy

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package g0901_1000.s0957_prison_cells_after_n_days;

// #Medium #Array #Hash_Table #Math #Bit_Manipulation
// #2022_02_18_Time_1_ms_(97.82%)_Space_42.1_MB_(31.56%)

import java.util.Arrays;

/**
 * 957 - Prison Cells After N Days\.
 *
 * Medium
 *
 * There are `8` prison cells in a row and each cell is either occupied or vacant.
 *
 * Each day, whether the cell is occupied or vacant changes according to the following rules:
 *
 * *   If a cell has two adjacent neighbors that are both occupied or both vacant, then the cell becomes occupied.
 * *   Otherwise, it becomes vacant.
 *
 * **Note** that because the prison is a row, the first and the last cells in the row can't have two adjacent neighbors.
 *
 * You are given an integer array `cells` where `cells[i] == 1` if the i^th cell is occupied and `cells[i] == 0` if the i^th cell is vacant, and you are given an integer `n`.
 *
 * Return the state of the prison after `n` days (i.e., `n` such changes described above).
 *
 * **Example 1:**
 *
 * **Input:** cells = [0,1,0,1,1,0,0,1], n = 7
 *
 * **Output:** [0,0,1,1,0,0,0,0]
 *
 * **Explanation:** The following table summarizes the state of the prison on each day:
 *
 * Day 0: [0, 1, 0, 1, 1, 0, 0, 1]
 *
 * Day 1: [0, 1, 1, 0, 0, 0, 0, 0]
 *
 * Day 2: [0, 0, 0, 0, 1, 1, 1, 0]
 *
 * Day 3: [0, 1, 1, 0, 0, 1, 0, 0]
 *
 * Day 4: [0, 0, 0, 0, 0, 1, 0, 0]
 *
 * Day 5: [0, 1, 1, 1, 0, 1, 0, 0]
 *
 * Day 6: [0, 0, 1, 0, 1, 1, 0, 0]
 *
 * Day 7: [0, 0, 1, 1, 0, 0, 0, 0]
 *
 * **Example 2:**
 *
 * **Input:** cells = [1,0,0,1,0,0,1,0], n = 1000000000
 *
 * **Output:** [0,0,1,1,1,1,1,0]
 *
 * **Constraints:**
 *
 * *   `cells.length == 8`
 * *   `cells[i]` is either `0` or `1`.
 * *   1 <= n <= 10⁹
**/
public class Solution {
    public int[] prisonAfterNDays(int[] cells, int n) {
        if (n == 0) {
            return cells;
        }
        int[] first = null;
        int[] prev = cells;
        int period;
        int day = 0;
        while (n > 0) {
            day++;
            n--;
            int[] next = getNextDay(prev);
            if (Arrays.equals(next, first)) {
                period = day - 1;
                n %= period;
            }
            if (day == 1) {
                first = next;
            }
            prev = next;
        }
        return prev;
    }

    private int[] getNextDay(int[] arr) {
        int[] ret = new int[8];
        for (int i = 1; i < 7; ++i) {
            if (arr[i - 1] == arr[i + 1]) {
                ret[i] = 1;
            }
        }
        return ret;
    }
}