LeetCode | Pascal's Triangle

題目

Given numRows, generate the first numRows of Pascal's triangle.

For example, given numRows = 5,
Return

[
     [1],
    [1,1],
   [1,2,1],
  [1,3,3,1],
 [1,4,6,4,1]
]

逐層構造，注意邊界條件。

代碼

import java.util.ArrayList;

public class PascalTriangle {
	public ArrayList> generate(int numRows) {
		ArrayList> results = new ArrayList>();
		if (numRows <= 0) {
			return results;
		}
		ArrayList firstRow = new ArrayList();
		firstRow.add(1);
		results.add(firstRow);
		ArrayList lastRow = firstRow;
		for (int i = 1; i < numRows; ++i) {
			ArrayList row = new ArrayList();
			row.add(1);
			for (int j = 1; j < i - 1; ++j) {
				row.add(lastRow.get(j - 1) + lastRow.get(j));
			}
			row.add(1);
			results.add(row);
			lastRow = row;
		}
		return results;
	}
}