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?

public class Solution {
    public ArrayList<Integer> getRow(int rowIndex) {
        ArrayList<Integer> res = new ArrayList<Integer>();
        int num [] = new int [rowIndex+2];
        num[0]=1;
        num[1] =1;
        
        for(int i=2;i<=rowIndex;i++){
            for(int j=i;j>=0;j--){
                if(j==i || j==0)
                    num[j] = 1;
                else{
                    num[j] = num[j]+num[j-1];
                }
            }
        }
        for(int i=0;i<rowIndex+1;i++){
            res.add(num[i]);
        }
        return res;
    }
}