Unique Paths
Description: A robot starts at the top-left of an m × n grid and can only move right or down. How many distinct paths lead to the bottom-right corner?
Example 1:
Input: m = 3, n = 7
Output: 28
Example 2:
Input: m = 3, n = 2
Output: 3
Constraints:
1 <= m, n <= 100
Recommended time and space complexity
Aim for O(m × n) time and O(n) space.
Hint 1
Any cell is reachable only from above or from the left.
Hint 2
So paths to a cell equal paths from above plus paths from the left.
Hint 3
Computing the current row needs only the previous one.