PrisonCellsAfterNDays.java

/*
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) {
        
       HashSet<String> set = new HashSet<>();
       int size = 0;
       
       //Patttern will start repeating after N iterations <= 2^6 = 64
       //6 cells with 2 possible values will have have 64 combinations
       for(int i=0; i<N; i++){
          
           int[] nextDayValue = nextDay(cells);
           String s = Arrays.toString(nextDayValue);
           
           if(!set.contains(s)){
               set.add(s);
               size++;
           }
           else{
               break;
           }
           cells = nextDayValue;
       }
        
       if(size > 0){
           N = N % size;
           for(int i=1; i<=N; i++){
               cells = nextDay(cells);
           }
       } 
       return cells; 
    }
    
    private int[] nextDay(int[] cells){
        int[] result = new int[cells.length];
        
        for(int i=1; i<cells.length-1; i++){
            if(cells[i-1] == cells[i+1]){
                result[i] = 1;
            }
        }
        
        return result;
    }
}