题目描述：
    image.png
    image.png
    解析：dp 从右下往左上推

    1. class Solution {
    2.         public int uniquePaths(int m, int n) {
    3.             if (m == 1 || n == 1) {
    4.                 return 1;
    5.             }
    6.             int[][] dp = new int[m][n];
    7.             for (int i = 0; i < m - 1; i++) {
    8.                 dp[i][n - 1] = 1;
    9.             }
    10.             for (int i = 0; i < n - 1; i++) {
    11.                 dp[m - 1][i] = 1;
    12.             }
    13.             int currRow = m - 2;
    14.             int currColum = n - 2;
    15.             while (currRow >= 0) {  //保证行不越界
    16.                 dp[currRow][currColum] = dp[currRow + 1][currColum] + dp[currRow][currColum + 1];
    17.                 currColum--;
    18.                 if (currColum < 0) {  //表示列越界
    19.                     currRow--;
    20.                     currColum = n - 2;
    21.                 }
    22.             }
    23.             return dp[0][0];
    24.         }
    25.     }