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.

 1 public class Solution {
 2     public List<List<Integer>> generate(int numRows) {
 3         List<List<Integer>> result = new ArrayList<List<Integer>>();
 4         if (numRows == 0)
 5             return result;
 6         List<Integer> prev = new ArrayList<Integer>();
 7         prev.add(1);
 8         result.add(prev);
 9         while (numRows > 1) {
10             List<Integer> current = new ArrayList<Integer>();
11             int length = prev.size();
12             current.add(1);
13             for (int i = 0; i < length - 1; i++)
14                 current.add(prev.get(i) + prev.get(i + 1));
15             current.add(1);
16             result.add(current);
17             prev = current;
18             numRows--;
19         }
20         return result;
21     }
22 }