Pascal's Triangle

Given an integer numRows, return the first numRows of Pascal's triangle.

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

 

Example 1:

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

Example 2:

Input: numRows = 1
Output: [[1]]

Solution

class Solution {

public List<List<Integer>> generate(int numRows) {

List<List<Integer>> triangle = new ArrayList<List<Integer>>();

// Base case; first row is always [1].

triangle.add(new ArrayList<>());

triangle.get(0).add(1);

for (int rowNum = 1; rowNum < numRows; rowNum++) {

List<Integer> row = new ArrayList<>();

List<Integer> prevRow = triangle.get(rowNum - 1);

// The first row element is always 1.

row.add(1);

// Each triangle element (other than the first and last of each row)

// is equal to the sum of the elements above-and-to-the-left and

// above-and-to-the-right.

for (int j = 1; j < rowNum; j++) {

row.add(prevRow.get(j - 1) + prevRow.get(j));

}

// The last row element is always 1.

row.add(1);

triangle.add(row);

}

return triangle;

}

}