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 }