Pascal's Triangle II 解答

Question

Given an index k, return the kth row of the Pascal's triangle.

For example, given k = 3,
Return [1,3,3,1].

Solution

Similar with Pascal's Triangle. Note that the index starts from 0.

 public class Solution {
     public List<Integer> getRow(int rowIndex) {
         List<Integer> result = new ArrayList<Integer>();
         List<Integer> prev = new ArrayList<Integer>();
         if (rowIndex < 0)
             return result;
         result.add(1);
         prev.add(1);
         while (rowIndex > 0) {
             result = new ArrayList<Integer>();
             result.add(1);
             int length = prev.size();
             for (int i = 0; i < length - 1; i++)
                 result.add(prev.get(i) + prev.get(i + 1));
             result.add(1);
             prev = result;
             rowIndex--;
         }
         return result;
     }
 }

We can also only use one list.

 public List<Integer> getRow(int rowIndex) {
     ArrayList<Integer> result = new ArrayList<Integer>();

     if (rowIndex < 0)
         return result;

     result.add(1);
     for (int i = 1; i <= rowIndex; i++) {
         for (int j = result.size() - 2; j >= 0; j--) {
             result.set(j + 1, result.get(j) + result.get(j + 1));
         }
         result.add(1);
     }
     return result;
 }