289.cpp

class Solution {
  // Runtime: 4 ms, faster than 83.08% of C++ online submissions for Game of
  // Life. Memory Usage: 8.5 MB, less than 97.14% of C++ online submissions for
  // Game of Life.
public:
  void gameOfLife(vector<vector<int>> &board) {
    int m = board.size(), n = m ? board[0].size() : 0;
    for (int i = 0; i < m; ++i) {
      for (int j = 0; j < n; ++j) {
        int count = 0;
        for (int I = max(i - 1, 0); I < min(i + 2, m); ++I)
          for (int J = max(j - 1, 0); J < min(j + 2, n); ++J)
            count += board[I][J] & 1;
        if (count == 3 || count - board[i][j] == 3)
          board[i][j] |= 2;
      }
    }
    for (int i = 0; i < m; ++i)
      for (int j = 0; j < n; ++j)
        board[i][j] >>= 1;
  }
};