You are given an m x n grid. A robot starts at the top-left corner (position [0, 0]) and wants to reach the bottom-right corner (position [m-1, n-1]).
The robot can only move right or down at each step.
Return the number of unique paths the robot can take to reach the destination.
+---+---+---+ | S | | | +---+---+---+ | | | | +---+---+---+ | | | E | +---+---+---+
Input: m = 3, n = 3 Output: 6 Explanation: There are 6 distinct paths from the top-left to the bottom-right on a 3x3 grid.
+---+---+---+---+---+---+---+ | S | | | | | | | +---+---+---+---+---+---+---+ | | | | | | | | +---+---+---+---+---+---+---+ | | | | | | | | +---+---+---+---+---+---+---+
Input: m = 3, n = 7 Output: 28 Explanation: There are 28 unique paths from top-left to bottom-right in a 3x7 grid.
m = 3, n = 7