LeetCode(118) Pascal's Triangle (Java)

標籤：arrays

題目如下：

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]
]

My Code：

// 225mspublic 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;    }}

小結分析:

1 上面的兩個錯誤，也是我正好不懂的關於java的文法的部分。

第一個錯誤，很好理解，抽象基類無法進行new的操作。

第二個錯誤，比較好玩， 搜到一個解釋，是這樣的:

假設這個寫法沒有錯誤，那麼可以寫成類似下面這個例子這樣的寫法:

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>

這將明顯是危險。That‘s clearly not type safe - you‘ve ended up with an apple in the collection of bananas.

LeetCode(118) Pascal's Triangle (Java)