题目：给定一个行数n，生成n行帕斯卡三角形

算法：逐层遍历数组

public class Solution {
    public List<List<Integer>> generate(int numRows) {
        if (numRows < 0) {
            return null;
        }
        
        List<List<Integer>> pascalTriangle = new ArrayList<List<Integer>>();
        for (int i=0; i<numRows; ++i) {
            if (i == 0) {
                List<Integer> currTriangle = new ArrayList<Integer>();
                currTriangle.add(1);
                pascalTriangle.add(i, currTriangle);
            } else {
                List<Integer> currTriangle = new ArrayList<Integer>();
                List<Integer> lastTriangle = pascalTriangle.get(i-1);
                
                int lastInteger = 0;
                for (int j=0; j<lastTriangle.size(); ++j) {
                    currTriangle.add(lastInteger + lastTriangle.get(j));
                    lastInteger = lastTriangle.get(j);
                }
                currTriangle.add(1);
                pascalTriangle.add(currTriangle);
            }
        }// end of for
        
        return pascalTriangle;
    }
}

Pascal's Triangle