Unique Paths

题目：

A robot is located at the top-left corner of a m x n grid (marked 'Start' in the diagram below).

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 (marked 'Finish' in the diagram below).

How many possible unique paths are there?

我的解法：

（1）算法思想：

根据m,n值，设一个矩阵matrix[m-1][n-1]，矩阵最上面一行和最左边一列的值都为1，其他位置元素的值等于相邻左边元素的值加上上边元素的值。这样，右下角元素的值就是可能有的路径数。

（2）代码如下：