LeetCode 119 Pascal's Triangle II

题：
https://leetcode.com/problems/pascals-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?

解法较容易，时间复杂度依然是0(n^2)，要实现空间复杂度得考虑在内层循环时从后往前看，这样的话上一行的结果不会被覆盖。

public class Solution {
public List<Integer> getRow(int rowIndex) {
if (rowIndex < 0) return null;
List<Integer> results = new ArrayList<Integer>(rowIndex + 1);
for (int row=0; row < rowIndex; row++) {
results.add(1);
for (int i=row; i>0; i--) {
results.set(i, results.get(i - 1) + results.get(i));
}
}
results.add(1); // add last 1
return results;
}
}