GameOfLife.java

// Time Complexity : O(m*n)
// Space Complexity : O(1)

class Solution {
    public void gameOfLife(int[][] board) {
        // edge case
        if (board == null || board.length == 0 || board[0].length == 0) {
            return;
        }
        // 1 - 0 2 (was alive and dead now)
        // 0 - 1 3 (was dead and alive now)
        int m = board.length;
        int n = board[0].length;
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                int count = countLives(board, i, j, m, n);
                if (board[i][j] == 1 && (count < 2 || count > 3)) { // rule 1 and rule 3
                    // died
                    board[i][j] = 2;
                }
                if (board[i][j] == 0 && count == 3) {// rule 4
                    // became alive
                    board[i][j] = 3;
                }
            }
        }
        // convert temporary 2 and 3 values back to what they originally were
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (board[i][j] == 2) {
                    board[i][j] = 0;
                }
                if (board[i][j] == 3) {
                    board[i][j] = 1;
                }
            }
        }
    }

    private int countLives(int[][] board, int i, int j, int m, int n) {
        // checking 8 directions cause diagonal is included
        int result = 0;
        int[][] dirs = { { 0, 1 }, { 0, -1 }, { 1, 0 }, { -1, 0 }, { 1, 1 }, { 1, -1 }, { -1, 1 }, { -1, -1 } };
        for (int[] dir : dirs) {
            int r = i + dir[0];
            int c = j + dir[1];
            if (r >= 0 && r < m && c >= 0 && c < n && (board[r][c] == 1 || board[r][c] == 2)) {
                result++;
            }

        }
        return result;
    }

}