LeetCode - Pascal's Triangle

題目：

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]
]

思路：

遞歸

package recursion;

import java.util.List;
import java.util.ArrayList;

public class PascalsTriangle {

    public List<List<Integer>> generate(int numRows) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();;
        List<Integer> row = new ArrayList<Integer>();
        if (numRows == 1) {         
            row.add(1);
            res.add(row);
        } else if (numRows > 1){
            res = generate(numRows - 1);
            List<Integer> lastRow = res.get(numRows - 2);
            int size = lastRow.size();
            for (int i = 0; i < size; ++i) {
                row.add((i > 0 ? lastRow.get(i - 1) : 0) + lastRow.get(i));
            }
            row.add(lastRow.get(size - 1));
            res.add(row);
        }
        return res;
    }
    
    public static void main(String[] args) {
        PascalsTriangle p = new PascalsTriangle();
        List<List<Integer>> res = p.generate(5);
        for (List<Integer> row : res) {
            for (int i : row)
                System.out.print(i + " ");
            System.out.println();
        }
    }

}