Leetcode - 289. Game of Life

Cell State Mapping:

Original State	New State	Transitional Value	Meaning
1 (Alive)	0 (Dead)	2	Cell was alive but will die.
0 (Dead)	1 (Alive)	3	Cell was dead but will become alive.
1 (Alive)	1 (Alive)	1	Cell remains alive.
0 (Dead)	0 (Dead)	0	Cell remains dead.

This clearly shows how each original state transitions using the intermediate values.

const countNeighbours = (r, c, board) => {
    const ROWS = board.length;
    const COLS = board[0].length;
    let count = 0;
    const directions = [
        [0, 1], [1, 0], [0, -1], [-1, 0],  // Horizontal and vertical directions
        [1, 1], [1, -1], [-1, 1], [-1, -1] // Diagonal directions
    ];

    for (const [dr, dc] of directions) {
        const nr = r + dr;
        const nc = c + dc;
        // Only count originally alive cells (1 or 2)
        if (nr >= 0 && nr < ROWS && nc >= 0 && nc < COLS) {
            if (board[nr][nc] === 1 || board[nr][nc] === 2) {
                count++;
            }
        }
    }

    return count;
};

var gameOfLife = function (board) {
    const ROWS = board.length;
    const COLS = board[0].length;

    // First pass: Mark cells with transitional states
    for (let r = 0; r < ROWS; r++) {
        for (let c = 0; c < COLS; c++) {
            const nei = countNeighbours(r, c, board); // Count live neighbors

            if (board[r][c] === 1) {
                // Alive cell: stays alive with 2 or 3 neighbors, else dies
                if (nei < 2 || nei > 3) {
                    board[r][c] = 2; // Alive → Dead
                }
            } else if (board[r][c] === 0) {
                // Dead cell: becomes alive if exactly 3 neighbors
                if (nei === 3) {
                    board[r][c] = 3; // Dead → Alive
                }
            }
        }
    }

    // Second pass: Finalize the board state
    for (let r = 0; r < ROWS; r++) {
        for (let c = 0; c < COLS; c++) {
            if (board[r][c] === 2) {
                board[r][c] = 0; // Mark dead
            } else if (board[r][c] === 3) {
                board[r][c] = 1; // Mark alive
            }
        }
    }
};