Prison Cells After N Days

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:

Example 2:


  1. cells.length == 8
  2. cells[i] is in {0, 1}
  3. 1 <= N <= 10^9

Brute Force:

The simplest way to solve this problem would be, iterate through all the columns and check if cell[i-1] == cell[i+1] then set the cell[i] as 1 else 0.

But this way the time would exceed if N is more than 1000

Optimized Solution:

How can we optimize it?
As after the first iteration first and the last cells will be fixed with 0, so left with only 6 cells - and we have 2 choices 0 or 1
Save the states and in well-known data structure HashSet and check if it does not already exist before adding and also keep a length variable to track the length. Once we found the cycle we will return N % length day state.

Better Approach:

I tried to find the pattern of the loop. Well, the length of the loop can be 1, 7, or 14. So once we enter the loop, every 14 steps must be the same state. The length of cells is even, so for any state, we can find a previous state. So all states are in a loop.

PS: I wasn’t able to solve this problem, and finally have to search for the solutions. I hope if next time I am facing this kind of problem I will be able to solve it.

Happy Coding!!!

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