[LeetCode] 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]
]

Solution

class Solution {
    public List<List<Integer>> generate(int numRows) {
        List<List<Integer>> res = new ArrayList<>();
        if (numRows < 1) return res;

        List<Integer> row = new ArrayList<>();
        for (int i = 0; i < numRows; i++) {
            row.add(0, 1);
            for (int j = 1; j < row.size()-1; j++) {
                row.set(j, row.get(j)+row.get(j+1));
            }
            res.add(new ArrayList<>(row));
        }
        return res;
    }
}