118. Pascal's Triangle

Problem:

Given a non-negative integer numRows, generate the first numRows of Pascal's triangle.

In Pascal's triangle, each number is the sum of the two numbers directly above it.

Example:

Input: 5
Output:
[
     [1],
    [1,1],
   [1,2,1],
  [1,3,3,1],
 [1,4,6,4,1]
]

Analysis:
see code
Solution:

class Solution {
    public List<List<Integer>> generate(int numRows) {
        List<List<Integer>> res = new ArrayList<>();
        List<Integer> prev = new ArrayList<>();
        for (int i = 1; i <= numRows; i++) {
            List<Integer> cur =  new ArrayList<>();
            for (int j = 0; j < i; j++) {
                if (j == 0 || j == i - 1) {
                    cur.add(1);
                } else {
                    cur.add(prev.get(j - 1) + prev.get(j));
                }
            }
            res.add(cur);
            prev = cur;
        }       
        return res;
    }
}