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;
     }
 }