PrisonCellsAfterNDays2.java

// Prison Cells After N Days

// Solution
// 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.)

// We describe the current state of the prison in the following way: cells[i] == 1 if the i-th cell is occupied, else cells[i] == 0.

// Given the initial state of the prison, return the state of the prison after N days (and 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]
 

// Note:

// cells.length == 8
// cells[i] is in {0, 1}
// 1 <= N <= 10^9

class Solution {
    public int[] prisonAfterNDays(int[] cells, int N) {
        Set<String> seen = new HashSet(); // to store state of cells to check if pattern repeats
        boolean cycle = false; //check if cycle repeats
        int len = 0;
        for (int i = 0; i < N; i++) {
            int[] next = nextDayState(cells); // we will be the next day state of cells in next
            String key = Arrays.toString(next); // convert Array to string for storing it in hashset
            if (seen.contains(key)) { // if this key already exists in hashset break the loop and set cycle as true
                cycle = true;
                break;
            }
            seen.add(key); // add the key to the hashset
            len++; // increase the length by 1
            cells = next; // set cells to next day state and repeat 
        }

        if (cycle)
            return prisonAfterNDays(cells, N % len);
        return cells;
    }

    private int[] nextDayState(int[] cells) {
        int[] next = new int[cells.length];
        for (int i = 1; i < 7; i++)
            next[i] = cells[i - 1] == cells[i + 1] ? 1 : 0;
        return next;
    }
}