LeetCode_118. Pascal's Triangle

118. Pascal's Triangle

Easy

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]
]

package leetcode.easy;

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

public class PascalSTriangle {
	@org.junit.Test
	public void test() {
		System.out.println(generate(5));
	}

	public List<List<Integer>> generate(int numRows) {
		List<List<Integer>> triangle = new ArrayList<List<Integer>>();

		// First base case; if user requests zero rows, they get zero rows.
		if (numRows == 0) {
			return triangle;
		}

		// Second base case; first row is always [1].
		triangle.add(new ArrayList<Integer>());
		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;
	}
}