289.Game_of_Life.java

class Solution {
    public void gameOfLife(int[][] board) {
        if (board == null || board.length == 0) return;
        int m = board.length, n = board[0].length;

        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                int lives = liveNeighbors(board, m, n, i, j);

                // In the beginning, every 2nd bit is 0;
                // So we only need to care about when will the 2nd bit become 1.
                if (board[i][j] == 1 && lives >= 2 && lives <= 3) {  
                    board[i][j] = 3; // Make the 2nd bit 1: 01 ---> 11
                }
                if (board[i][j] == 0 && lives == 3) {
                    board[i][j] = 2; // Make the 2nd bit 1: 00 ---> 10
                }
            }
        }

        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                board[i][j] >>= 1;  // Get the 2nd state.
            }
        }
    }   

    public int liveNeighbors(int[][] board, int m, int n, int i, int j) {
        int lives = 0;
        for (int x = Math.max(i - 1, 0); x <= Math.min(i + 1, m - 1); x++) {
            for (int y = Math.max(j - 1, 0); y <= Math.min(j + 1, n - 1); y++) {
                lives += board[x][y] & 1;
            }
        }
        lives -= board[i][j] & 1;
        return lives;
    }
}



class Solution {
    public void gameOfLife(int[][] board) {
        int m = board.length;
        int n = board[0].length;
        
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                int lives = 0;
                
                // scan 3x3 region around board[j,i]
                for (int y = Math.max(0, i-1); y <= Math.min(m-1, i+1); ++y) {
                    for(int x = Math.max(0, j-1); x <= Math.min(n-1, j+1); ++x) {
                        // use & 1 to avoid updated cell status
                        lives += board[y][x] & 1;
                    }
                }
                
                if (lives == 3 || lives - board[i][j] == 3) {
                    // updates the second last bit
                    board[i][j] |= 0b10;
                }
            }
        }
        
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                board[i][j] >>= 1;
            }
        }
    
  }
}