LeetMotion
There is a robot on an m x n grid. The robot is initially located at the top-left corner (i.e., grid[0][0]). The robot tries to move to the bottom-right corner (i.e., grid[m - 1][n - 1]).
The robot can only move either down or right at any point in time. Given the two integers m and n, return the number of possible unique paths that the robot can take to reach the bottom-right corner.
Instead of a full 2D matrix, we only need to keep track of the row below the current row since the robot can only move down and right. We iterate from the bottom row upwards, and from right to left.
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