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Unique Paths - Leetcode 62 - Python

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.


The test cases are generated so that the answer will be less than or equal to 2 * 109. 



class Solution:
    def uniquePaths(self, m: int, n: int) -> int:
        r = [1]*n
        
        for i in range(m-1):
            nr = [1] * n
            for j in range(n-2, -1, -1):
                nr[j] = nr[j+1] + r[j]
            r = nr
			
        return r[0]


Explaination :






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