[Swift]LeetCode251.展平二维向量 $ Flatten 2D Vector

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Implement an iterator to flatten a 2d vector.

For example,
Given 2d vector =

[
  [1,2],
  [3],
  [4,5,6]
]

By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,2,3,4,5,6].

Hint:

1. How many variables do you need to keep track?
2. Two variables is all you need. Try with x and y.
3. Beware of empty rows. It could be the first few rows.
4. To write correct code, think about the invariant to maintain. What is it?
5. The invariant is x and y must always point to a valid point in the 2d vector. Should you maintain your invariant ahead of time or right when you need it?
6. Not sure? Think about how you would implement hasNext(). Which is more complex?
7. Common logic in two different places should be refactored into a common method.

Follow up:
As an added challenge, try to code it using only iterators in C++ or iterators in Java.

---

实现迭代器以展平二维向量。

例如，

给定的二维矢量=

[
  [1,2],
  [3],
  [4,5,6]
]

经过重复调用next直到hasnext返回false，next返回的元素顺序应该是：[1,2,3,4,5,6]。

提示：

1. 您须要跟踪多少个变量？
2. 你只须要两个变量。尝试使用x和y。
3. 小心空行。多是前几排。
4. 要编写正确的代码，请考虑要维护的不变量。这是怎么一回事？
5. 不变量是x，y必须始终指向二维向量中的有效点。您应该提早保持不变仍是在须要时保持不变？
6. 不肯定？考虑如何实现hasNext（）。哪一个更复杂？
7. 两个不一样地方的公共逻辑应该重构为一个公共方法。

跟进：

做为一个额外的挑战，尝试用C++中的迭代器或Java中的迭代器对其进行编码。

---

 1 class WordDistance {
 2     var x:Int
 3     var y:Int
 4     var v:[[Int]] = [[Int]]()
 5     init(_ vec2d:[[Int]]) {
 6         // perform some initialization here
 7         v = vec2d
 8         x = 0
 9         y = 0
10     }
11     
12     func next() -> Int
13     {
14         var num:Int = v[x][y]
15         y += 1
16         return num
17     }
18     
19     func hasNext() -> Bool
20     {
21         while (x < v.count && y == v[x].count) {
22             x += 1
23             y = 0
24         }
25         return x < v.count
26     }
27 }