The idea is simple just like other solutions, and the diffrence is I used only one nested loop instead of two. For each point, there are previous-neighbors:

[i-1,j-1].....[i-1,j]..... [ i-1, j+1]

[i, j-1]

and post-neighbors:

................................[i, j+1]

[i+1, j-1]....[i+1, j].....[i+1,j+1]

We loop through board, find board[i][j]'s totalNeighbor = previous + post, and if board[i][j] == 1, we update all its post-neighbors: ++ for those >=1, and -- for those <= 0. After this operation, each board[i][j] record its previous neighbors and we only need to find its post neighbors, thus one loop.

'''

public class Solution {

```
public void gameOfLife(int[][] board) {
if (board == null || board.length == 0) {
return;
}
int row = board.length, col = board[0].length;
for (int i = 0; i < row; i++) {
for (int j = 0; j < col; j++) {
// board[i][j] > 0 means there was an '1' initially
if (board[i][j] > 0) {
int totalNeighbors = board[i][j] - 1 + postNeighbors(board, i, j, row, col);
if (totalNeighbors == 2 || totalNeighbors == 3) {
board[i][j] = 1;
} else {
board[i][j] = 0;
}
// And we update its post neighbors
updatePostNeighbors(board, i, j, row, col);
continue;
}
// board[i][j] <= 0 means there was a '0' initially
if (board[i][j] <= 0) {
int totalNeighbors = -board[i][j] + postNeighbors(board, i, j, row, col);
if (totalNeighbors == 3) {
board[i][j] = 1;
} else {
board[i][j] = 0;
}
}
}
}
}
private void updatePostNeighbors(int[][] board, int i, int j, int row, int col) {
if (j+1 < col) {
if (board[i][j+1] >= 1) {
board[i][j+1]++;
} else {
board[i][j+1]--;
}
}
if (i + 1 < row) {
if (j-1 >= 0) {
if (board[i+1][j-1] >= 1) {
board[i+1][j-1]++;
} else {
board[i+1][j-1]--;
}
}
board[i+1][j] = board[i+1][j] >= 1? board[i+1][j] + 1 : board[i+1][j] - 1;
if (j+1 < col) {
board[i+1][j+1] = board[i+1][j+1] >= 1? board[i+1][j+1] + 1 : board[i+1][j+1] - 1;
}
}
}
private int postNeighbors(int[][] b, int i, int j, int row, int col) {
int n = 0;
if (j+1 < col && b[i][j+1] >= 1) n++;
if (i + 1 < row) {
if (j-1 >= 0 && b[i+1][j-1] >= 1) n++;
if (b[i+1][j] >= 1) n++;
if (j+1 < col && b[i+1][j+1] >= 1) n++;
}
return n;
}
```

}

'''