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;
    }
}