简单 dp，注意follow up

class Solution {
    public int uniquePaths(int m, int n) {
        int[][] dp = new int[m][n];

        for (int i = 0; i < m; i++){
            dp[i][0] = 1;
        }

        for (int j = 0; j < n; j++){
            dp[0][j] = 1;
        }

        for (int i = 1; i < m; i++){
            for (int j = 1; j < n; j++){
                dp[i][j] = dp[i - 1][j] + dp[i][j - 1];
            }
        }

        return dp[m - 1][n - 1];
    }
}