杨辉三角

题目描述：
给定一个非负整数 numRows，生成杨辉三角的前 numRows 行。
在杨辉三角中，每个数是它左上方和右上方的数的和。

示例：
输入: 5
输出:
[
[1],
[1,1],
[1,2,1],
[1,3,3,1],
[1,4,6,4,1]
]

Java代码：

class Solution {
    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<>());
        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;
    }
}