62\. Unique Paths

Medium

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?

**Example 1:**

![](https://assets.leetcode.com/uploads/2018/10/22/robot_maze.png)

**Input:** m = 3, n = 7

**Output:** 28 

**Example 2:**

**Input:** m = 3, n = 2

**Output:** 3

**Explanation:**

    From the top-left corner, there are a total of 3 ways to reach the bottom-right corner:
    1. Right -> Down -> Down
    2. Down -> Down -> Right
    3. Down -> Right -> Down 

**Example 3:**

**Input:** m = 7, n = 3

**Output:** 28 

**Example 4:**

**Input:** m = 3, n = 3

**Output:** 6 

**Constraints:**

*   `1 <= m, n <= 100`
*   It's guaranteed that the answer will be less than or equal to <code>2 * 10<sup>9</sup></code>.