leetcode -- Pascal's Triangle

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]
]

[解题思路]

第三行开始，每行除边界元素外，每个元素ai都是由ai-1 + ai构成

public class Solution {
    public ArrayList<ArrayList<Integer>> generate(int numRows) {
        // Start typing your Java solution below
        // DO NOT write main() function
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if(numRows <= 0){
            return result;
        }
        // row 1
        ArrayList<Integer> one = new ArrayList<Integer>();
        one.add(1);
        result.add(one);
        if(numRows == 1){
            return result;
        }
        // row 2
        ArrayList<Integer> two = new ArrayList<Integer>();
        two.add(1);
        two.add(1);
        result.add(two);
        if(numRows == 2){
            return result;
        }
        for(int i = 3; i <= numRows; i++){
            int resultSize = result.size();
            ArrayList<Integer> last = result.get(resultSize - 1);
            ArrayList<Integer> cur = new ArrayList<Integer>();
            for(int j = 1; j <= i; j++){
                if(j == 1 || j == i){
                    cur.add(1);
                } else {
                    int num = last.get(j - 1) + last.get(j - 2);
                    cur.add(num);
                }
            }
            result.add(cur);
        }
        return result;
    }
}