LeetCode(118) Pascal's Triangle (Java)

题目如下：

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]

]

我的代码：

// 225ms
public class Solution {
    public List<List<Integer>> generate(int numRows) {
        
        
        //ERROR1: List is abstract; cannot be instantiated
        //List<List<Integer>> lists = new List<List<Integer>> ();
        
        //ERROR2: fail to ensure type safe
        //http://stackoverflow.com/questions/5763750/why-we-cant-do-listparent-mylist-arraylistchild
        //List<List<Integer>> lists = new ArrayList<List<Integer>> ();
        
        List<List<Integer>> lists = new ArrayList<ArrayList<Integer>> ();
        if (numRows <= 0) return lists;
        List<Integer> list1 = new ArrayList<Integer> ();
        list1.add(1);
        lists.add(list1);
        if (numRows == 1) return lists;
        List<Integer> oldList = null;
        for (int i = 2; i <=numRows; ++i) {
            List<Integer> newList = new ArrayList<Integer> ();
            newList.add(1);
            newList.add(1);
            for (int j = 1; j < i - 1; ++j) {
                newList.add(j, oldList.get(j) + oldList.get(j - 1));
            }
            lists.add(newList);
            oldList = newList;
        }
        return lists;
    }
}

小结分析:

1 上面的两个错误，也是我正好不懂的关于java的语法的部分。

第一个错误，很好理解，抽象基类无法进行new的操作。

第二个错误，比较好玩， 搜到一个解释，是这样的:

假设这个写法没有错误，那么可以写成类似下面这个例子这样的写法:

ArrayList<Banana> bananas = new ArrayList<Banana>();
List<Fruit> fruit = bananas;
fruit.add(new Apple());

Banana banana = bananas.get(0); //ERROR! not type safe<span style="white-space:pre">	</span>

这将明显是危险。That's clearly not type safe - you've ended up with an apple in the collection of bananas.