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?

滚动数组 + 从后往前计算 避免重复

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