Pascal's Triangle 解答

Question

Given numRows, generate the first numRows of Pascal's triangle.

For example, given numRows = 5,
Return

[

     [1],

    [1,1],

   [1,2,1],

  [1,3,3,1],

 [1,4,6,4,1]

]

Solution

Key to the solution is to use two arrays, one for current list, one for previous array.

 public class Solution {

     public List<List<Integer>> generate(int numRows) {

         List<List<Integer>> result = new ArrayList<List<Integer>>();

         if (numRows == 0)

             return result;

         List<Integer> prev = new ArrayList<Integer>();

         prev.add(1);

         result.add(prev);

         while (numRows > 1) {

             List<Integer> current = new ArrayList<Integer>();

             int length = prev.size();

             current.add(1);

             for (int i = 0; i < length - 1; i++)

                 current.add(prev.get(i) + prev.get(i + 1));

             current.add(1);

             result.add(current);

             prev = current;

             numRows--;

         }

         return result;

     }

 }