289. Game of Life

package LeetCode_289

/**
 * 289. Game of Life
 * https://leetcode.com/problems/game-of-life/
 * According to the Wikipedia's article:
 * "The Game of Life, also known simply as Life, is a cellular automaton devised by the British mathematician John Horton Conway in 1970."
Given a board with m by n cells, each cell has an initial state live (1) or dead (0).
Each cell interacts with its eight neighbors (horizontal, vertical, diagonal) using the following four rules (taken from the above Wikipedia article):
1. Any live cell with fewer than two live neighbors dies, as if caused by under-population.
2. Any live cell with two or three live neighbors lives on to the next generation.
3. Any live cell with more than three live neighbors dies, as if by over-population.
4. Any dead cell with exactly three live neighbors becomes a live cell, as if by reproduction.
Write a function to compute the next state (after one update) of the board given its current state.
The next state is created by applying the above rules simultaneously to every cell in the current state, where births and deaths occur simultaneously.
Example:
Input:
[
[0,1,0],
[0,0,1],
[1,1,1],
[0,0,0]
]
Output:
[
[0,0,0],
[1,0,1],
[0,1,1],
[0,1,0]
]
 * */
class Solution {
    /*
    * solution: simulation, scan board:
    * 1. if need set die to live, mark current position to -1,
    * 2. if need set live to die, mark current position to 2,
    * Time: O(m*n), Space: O(1)
    * */
    val directions = arrayOf(
        //x,y
        intArrayOf(0, 1),//up
        intArrayOf(1, 0),//right
        intArrayOf(0, -1),//down
        intArrayOf(-1, 0),//left
        intArrayOf(-1, 1),//up,left
        intArrayOf(1, 1),//up,right
        intArrayOf(-1, -1),//down,left
        intArrayOf(1, -1)//down,right
    )

    fun gameOfLife(board: Array<IntArray>): Unit {
        val m = board.size
        val n = board[0].size
        for (i in 0 until m) {
            for (j in 0 until n) {
                //need set to live: Any dead cell with exactly three live neighbors becomes a live cell
                if (board[i][j] == 0) {
                    val liveCount = countLive(board, i, j)
                    if (liveCount == 3) {
                        board[i][j] = -1
                    }
                }
                //need set to die:
                //1. Any live cell with more than three live neighbors dies
                //2. Any live cell with fewer than two live neighbors dies
                if (board[i][j] == 1) {
                    val liveCount = countLive(board, i, j)
                    if (liveCount > 3 || liveCount < 2) {
                        board[i][j] = 2
                    }
                }
            }
        }
        update(board)
    }

    private fun countLive(board: Array<IntArray>, x: Int, y: Int): Int {
        var count = 0
        for (dir in directions) {
            val newX = x + dir[0]
            val newY = y + dir[1]
            if (newX < 0 || newY < 0 || newX >= board.size || newY >= board[0].size) {
                continue
            }
            if (board[newX][newY] == 1 || board[newX][newY] == 2) {
                count++
            }
        }
        return count
    }

    private fun update(board: Array<IntArray>) {
        val m = board.size
        val n = board[0].size
        for (i in 0 until m) {
            for (j in 0 until n) {
                if (board[i][j] == -1) {
                    board[i][j] = 1
                }
                if (board[i][j] == 2) {
                    board[i][j] = 0
                }
            }
        }
    }
}