Leetcode: Pascal's Triangle II

Given an index k, return the kth row of the Pascal's triangle.

For example, given k = 3,
Return [1,3,3,1].

Note:
Could you optimize your algorithm to use only O(k) extra space?

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Note: O(k) means we can only use in-place way:

method: from right to left

eg.:

1   4   6  4  1 0 previous line

                            

0   0  0  0   0  1 now calculate in place from right to left 

0   0  0  0   5  1

0   0  0  10  5  1

0   0  10  10  5  1

0   5  10  10  5  1

1   5  10  10  5  1