119. Pascal's Triangle II

Given a non-negative index k where k ≤ 33, return the k^th index row of the Pascal's triangle.

Note that the row index starts from 0.

^{In Pascal's triangle, each number is the sum of the two numbers directly above it.}

Example:

Input: 3
Output: [1,3,3,1]

Follow up:

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