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?

Original algorithm (recursive approach) can be referred to [leet code] Pascal's Triangle.

Idea of using only O(k) extra space is reusing the same array list to form the current row of numbers. Iterations will be from 0th row ([1]) to kth row. And for each iteration, we need to form the current base on the previous row.

For example, in 3th row, we have [1,3,3,1]. So in the iteration of 4th row, we need only add 1 at the beginning of this array list, overwritten the 2nd element to 1+3=4, 3rd element to 3+3=6, 4th element to 3+1=4, and keep the last element remain unchanged. Then we would have all the numbers of 4th row, i.e.[1,4,6,4,1].

 1 public class Solution {
 2     public List<Integer> getRow(int rowIndex) {
 3         ArrayList<Integer> res = new ArrayList<Integer>();
 4         if (rowIndex < 0) return res;
 5         for (int i=0; i<=rowIndex; i++) {
 6             for (int j=i; j>=0; j--) {
 7                 if (j == i) res.add(1);
 8                 else if (j > 0) {
 9                     res.set(j, res.get(j-1)+res.get(j));
10                 }
11             }
12         }
13         return res;
14     }
15 }

 1 public class Solution {
 2     public ArrayList<Integer> getRow(int rowIndex) {
 3         ArrayList<Integer> list = new ArrayList<Integer>();
 4         list.add(1);
 5         int prev, current;
 6         for (int i = 0; i <= rowIndex; i++) {
 7             prev = 1;
 8             for (int j = 0; j <= i; j++) {
 9                 if (j == 0) continue;
10                 if (j == i) list.add(1);
11                 else {
12                     current = list.get(j);
13                     list.set(j, current + prev);
14                     prev = current;
15                 }
16             }
17         }
18         return list;
19     }
20 }

 1 public ArrayList<Integer> getRow(int rowIndex) {
 2     ArrayList<Integer> res = new ArrayList<Integer>();
 3     if(rowIndex<0)
 4         return res;
 5     res.add(1);
 6     for(int i=1;i<=rowIndex;i++)
 7     {
 8         for(int j=res.size()-2;j>=0;j--)
 9         {
10             res.set(j+1,res.get(j)+res.get(j+1));
11         }
12         res.add(1);
13     }
14     return res;
15 }