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 .

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
     
       getRow(int rowIndex) { 3         ArrayList
      
        res = new ArrayList
       
        (); 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
     
       getRow(int rowIndex) { 3         ArrayList
      
        list = new ArrayList
       
        (); 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
     
       getRow(int rowIndex) { 2     ArrayList
      
        res = new ArrayList
       
        (); 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 }