因为限定了只能下和右，所以第一行和第一列边界就是直着走

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