Description
A robot is located at the top-left corner of a m x n grid.
The robot can only move either down or right at any point in time. The robot is trying to reach the bottom-right corner of the grid.
How many possible unique paths are there?
m and n will be at most 100.
Example
Example 1:
Input: n = 1, m = 3
Output: 1
Explanation: Only one path to target position.
Example 2:
Input: n = 3, m = 3
Output: 6
Explanation:
D : Down
R : Right
1) DDRR
2) DRDR
3) DRRD
4) RRDD
5) RDRD
6) RDDR
思路：
坐标型动态规划，当前点的路径数等于前序点路径数之和，这里有用hashmap和list实现的两种方法，需要注意初始化时的浅拷贝问题。
代码：
