二维dp，轻轻松松

class Solution {
public:
    int getMaxValue(vector<vector<int>>& grid) {
        size_t m=grid.size();
        if(m==0){
            return 0;
        }
        size_t n=grid[0].size();
        vector<vector<int> > dp(m,vector<int>(n,0));
        for(int i=0;i<m;i++){
            for(int j=0;j<n;j++){
                if(i==0&&j==0){
                    dp[i][j]=grid[i][j];
                    continue;
                }
                if(i==0){
                    dp[i][j]=grid[i][j]+dp[i][j-1];
                    continue;
                }
                if(j==0){
                    dp[i][j]=grid[i][j]+dp[i-1][j];
                    continue;
                }
                dp[i][j]=max(dp[i-1][j],dp[i][j-1])+grid[i][j];
            }
        }
        return dp[m-1][n-1];
    }
};