http://oj.leetcode.com/problems/pascals-triangle/

Pascal'sTriangle.java

public class Solution {
    public ArrayList<ArrayList<Integer>> generate(int numRows) {
        ArrayList<Integer> ret = new ArrayList<Integer>();
        ArrayList<Integer> ret_temp = new ArrayList<Integer>();
        ArrayList<ArrayList<Integer>> rn = new ArrayList<ArrayList<Integer>>();
        if (numRows == 0) return rn;
        ret_temp.add(1);
        rn.add(ret_temp);
        if (numRows == 1) return rn;
        ret.add(1);
        ret.add(1);
        rn.add(ret);
        if (numRows == 2) return rn;
        for (int i = 3; i <= numRows; i++){
            ret_temp = new ArrayList<Integer>();
            ret_temp = ret;
            ret = new ArrayList<Integer>();
            ret.add(1);
            for (int j = 0; j < ret_temp.size() - 1; j++){
                int value = ret_temp.get(j) + ret_temp.get(j + 1);
                ret.add(value);
            } 
            ret.add(1);
            rn.add(ret);
        }
        return rn;
    }
}

Java中ArrayList的add方法
添加的是对象的引用

Example:
ArrayList A = new ArrayList();
ArrayList<ArrayList> B = new ArrayList<ArrayList>();

A.add(1);
B.add(A);//B: [[1]]
A.add(2);//B: [[1, 2]]