Python solution with explanation.

  • 0

    This approach is pretty much the same as others but I've chosen to use positive/negative to "encode" additional information. Nice thing about this is that you can use mod operator which only cares about the absolute values. I also handle my bounds a bit differently than most.

        def gameOfLife(self, board):
            :type board: List[List[int]]
            :rtype: void Do not return anything, modify board in-place instead.
            num_rows, num_cols = len(board), len(board[0])
            for r in range(num_rows):
                is_firstr = not r
                is_lastr = (r == num_rows-1)
                for c in range(num_cols):
                    is_firstc = not c
                    is_lastc = (c == num_cols-1)
                    # Inclusive bounds.
                    rstart, rend = r-1, r+1
                    cstart, cend = c-1, c+1
                    # Update bounds if we are on edge.
                    if is_firstr: rstart = r
                    if is_lastr:  rend = num_rows-1
                    if is_firstc: cstart = c
                    if is_lastc:  cend = num_cols-1
                    # If a cell is 0, and it needs to live, assign -1.
                    # If a cell is 0, and it needs to remain dead, assign -2.
                    # If a cell is 1, and it needs to die, assign 2.
                    # If a cell is 1, and it needs to remain live, assign 1.
                    total_live = 0
                    for ir in range(rstart, rend+1):
                        total_live += sum(1 if x > 0 else 0 for x in board[ir][cstart:cend+1])
                    # Check current cell value and assign new value.
                    if board[r][c]:
                        # Previously summed 3x3 block but current cell needs to be omitted.
                        total_live -= 1
                        if total_live < 2 or total_live > 3:
                            board[r][c] = 2
                            board[r][c] = 1
                        if total_live == 3:
                            board[r][c] = -1
                            board[r][c] = -2
            # Normalize to 1s and 0s.           
            for r in range(num_rows):
                for c in range(num_cols):
                    if board[r][c] % 2:
                        board[r][c] = 1
                        board[r][c] = 0

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