PascalsTriangle.java

package leetcode;

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

/**
 * @author eko
 * @date 2018/10/29 10:47 AM
 *
 * 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]
 * ]
 */
public class PascalsTriangle {
    public static void main(String[] args) {
        List<List<Integer>> res = new PascalsTriangle().generate(5);
        System.out.println(res);
    }

    public List<List<Integer>> generate(int numRows) {
        List<List<Integer>> res = new ArrayList<>();
        for (int i = 0; i < numRows; i++) {
            List<Integer> row = new ArrayList<>();
            List<Integer> lastRow = new ArrayList<>();
            if (i != 0) {
                lastRow = res.get(i - 1);
            }
            for (int j = 0; j <= i; j++) {
                if (j == 0 || j == i) row.add(1);
                else row.add(lastRow.get(j - 1) + lastRow.get(j));
            }
            res.add(row);
        }
        return res;
    }
}