死->活：-1，死->死：0，活->活：1，活->死：2
这样就可以同时存储变化前后的状态了。

class Solution {
public:
    void gameOfLife(vector<vector<int>>& board) {
        for(int i=0;i<board.size();++i)
            for(int j=0;j<board[0].size();++j)
                _gameOfLife(board,i,j);
        for(int i=0;i<board.size();++i)
            for(int j=0;j<board[0].size();++j){
                if(board[i][j] == 2)
                    board[i][j] = 0;
                else if(board[i][j] == -1)
                    board[i][j] = 1;
            }
    }
    void _gameOfLife(vector<vector<int>>& board,int x,int y) {
        int live = 0;
        if(x>0)
        {
            if(board[x-1][y]>0)//左
                ++live;
            if(y>0 && board[x-1][y-1]>0)//左上
                ++live;
            if(y<board[0].size()-1 && board[x-1][y+1]>0)//左下
                ++live;
        }
        if(x<board.size()-1)
        {
            if(board[x+1][y]>0)//右
                ++live;
            if(y>0 && board[x+1][y-1]>0)//右上
                ++live;
            if(y<board[0].size()-1 && board[x+1][y+1]>0)//右下
                ++live;
        }
        if(y >0 && board[x][y-1]>0)//上
            ++live;
        if(y < board[0].size()-1 && board[x][y+1]>0)//下
            ++live;
        if(board[x][y] == 1)
        {
            if(live<2||live>3)
                board[x][y] = 2;
        }
        else if(live == 3)
            board[x][y] = -1;
    }
};