class Solution {
public:
    int getMaxValue(vector<vector<int>>& grid) {
        int n = grid.size();
        int m = grid[0].size();
        //第一列
        for(int i = 1; i < n; i++)
            grid[i][0] += grid[i - 1][0];
        //第一行
        for(int j = 1; j < m; j++)
        //只能从上面过来
            grid[0][j] += grid[0][j - 1];
        for(int i = 1; i < n; i++)
        //只能从左边过来
            for(int j = 1; j < m; j++)
            //每个格子只从上面过来或者左边过来,从中选择价值大的
                grid[i][j] += max(grid[i - 1][j], grid[i][j - 1]);
        return grid[n - 1][m - 1];
    }
};