Create 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?(用公式求组合，然后第一次遇到int表示不了的大整数的情况。 long转int：Integer.parseInt(l+""); int转long: l = i;)