class Solution {
public:
    void gameOfLife(vector<vector<int>>& board) {
        if(board.empty()) return;
        int n = board.size();
        int m = board[0].size();
        //8个遍历方向
        vector<vector<int>> dires = {{1, 0}, {0,1}, {-1, 0}, {0, -1}, {-1, -1}, {1, 1}, {-1, 1}, {1, -1}};
        //标记当前节点是否改变过
        vector<vector<bool>> flag(n, vector<bool>(m, false));
        //获取一个节点附近的活节点
        auto get_sum = [&](int x, int y) {
            int sum = 0;
            for(const auto& dir : dires) {
                auto next_x = x + dir[0];
                auto next_y = y + dir[1];
                if (next_x < n && next_y < m && next_x >= 0 && next_y >= 0) {
                    if(board[next_x][next_y] == 1 && flag[next_x][next_y] == false) ++sum;
                    if(board[next_x][next_y] == 0 && flag[next_x][next_y] == true)  ++sum;
                }
            }
            return sum;
        };
        //遍历整个图
        for(int i = 0; i < n; ++i) {
            for(int j = 0; j < m; ++j) {
                auto sum = get_sum(i, j); //当前节点附近的活结点
                //根据题目规则写出
                if(board[i][j] == 1) {
                    if(sum < 2 || sum > 3) board[i][j] = 0, flag[i][j] = true;
                }
                else {
                    if(sum == 3) board[i][j] = 1, flag[i][j] = true;
                }
            }
        }
    }
};