Leetcode(Path): Unique paths

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?

Above is a 3 x 7 grid. How many possible unique paths are there?

Note: m and n will be at most 100.

Solution:

In order to obtain O(n) we just keep the last row and the actual one

class Solution:
    # @return an integer
    def uniquePaths(self, m, n):
        for row in range(0,m):
            actualRow = []
            for col in range(0,n):
                if row ==0 and col==0:
                    actual = 1
                else:
                    actual = 0;
                if row>0:
                    actual += previousRow[col]
                if col>0:
                    actual += actualRow[col-1]
                actualRow.append(actual)
            previousRow= actualRow
        return actualRow[n-1]