957. 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:

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:

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

Rust Solution

struct Solution;

use std::collections::HashMap;

impl Solution {
    fn prison_after_n_days(mut cells: Vec<i32>, mut n: i32) -> Vec<i32> {
        let mut hm: HashMap<Vec<i32>, i32> = HashMap::new();
        while n > 0 {
            hm.insert(cells.to_vec(), n);
            let mut next = vec![0; 8];
            for i in 1..7 {
                next[i] = 1 - (cells[i - 1] ^ cells[i + 1]);
            }
            cells = next;
            n -= 1;
            if let Some(m) = hm.get(&cells) {
                n %= m - n;
            }
        }
        cells
    }
}

#[test]
fn test() {
    let cells = vec![0, 1, 0, 1, 1, 0, 0, 1];
    let n = 7;
    let res = vec![0, 0, 1, 1, 0, 0, 0, 0];
    assert_eq!(Solution::prison_after_n_days(cells, n), res);
    let cells = vec![1, 0, 0, 1, 0, 0, 1, 0];
    let n = 1_000_000_000;
    let res = vec![0, 0, 1, 1, 1, 1, 1, 0];
    assert_eq!(Solution::prison_after_n_days(cells, n), res);
}

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