LeetCode OJ 119. 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?

【思路】

我们为了满足空间复杂度的要求，我们新建两个ArrayList，一个负责存储上一个Pascal行的结果，一个根据上一个Pascal行得出当前Pascal行的结果，这个过程循环进行，代码如下：

 1 public class Solution {
 2     public List<Integer> getRow(int rowIndex) {
 3         List<Integer> prelist = new ArrayList<>();
 4         prelist.add(1);
 5         if(rowIndex <= 0) return prelist;
 6         List<Integer> curlist = new ArrayList<>();
 7         for(int i = 1; i <= rowIndex; i++){
 8             curlist.add(1);
 9             for(int j = 0; j < prelist.size() - 1; j++){
10                 curlist.add(prelist.get(j) + prelist.get(j+1));
11             }
12             curlist.add(1);
13             List<Integer> temp = prelist;
14             prelist = curlist;
15             curlist = temp;
16             curlist.clear();
17         }
18         return prelist;
19     }
20 }

LeetCode OJ 119. Pascal's Triangle II