# very simple to understand 3 - pass O(mn) time and O(1) space

• ``````class Solution(object):
def gameOfLife(self, board):
"""
:type board: List[List[int]]
:rtype: void Do not return anything, modify board in-place instead.
"""
m = len(board)
n = len(board[0]) if m > 0 else 0
if m and n:
# mark which are alive and which are dead, +ve alive, -ve dead
# also mark how many neighbors are alive in each cell
for i in range(m):
for j in range(n):
neighbors = [(i-1,j-1),(i+1,j+1),(i-1,j+1),(i+1,j-1),(i-1,j),(i+1,j),(i,j-1),(i,j+1)]
live      = 0
for x,y in neighbors:
if 0 <= x < m and 0 <= y < n:
if board[x][y] > 0:
live += 1
if live:
board[i][j] = live if board[i][j] > 0 else -live

# kill or liven the cells depending on rules
for i in range(m):
for j in range(n):
if board[i][j] > 0:
if board[i][j] < 2 or board[i][j] > 3 :
board[i][j] = -board[i][j]
else:
if abs(board[i][j]) == 3:
board[i][j] = -board[i][j]

# positive becomes 1 and negative becomes 0
for i in range(m):
for j in range(n):
if board[i][j] > 0:
board[i][j] = 1
else:
board[i][j] = 0``````

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