[LeetCode251]Flatten 2D Vector

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:

＊How many variables do you need to keep track?

＊Two variables is all you need. Try with x and y.

＊To write correct code, think about the invariant to maintain. What is it?

＊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?

＊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.

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根據提示：使用兩個iterator，一個指向2d vector的begin， 另一個指向2dvector的end（即在例子中一個指1另一個指在6的後面） 再利用一個int j來指向具體的column。

class Vector2D {
    vector<vector<int>> :: iterator i, iEnd;
    int j = 0;
public:
    Vector2D(vector<vector<int>>& vec2d) {
        i = vec2d.begin();
        iEnd = vec2d.end();
    }

    int next() {
        hasNext();
        return (*i)[j++];
    }

    bool hasNext() {
        while(i != iEnd && j == (*i).size()){
            ++i;
            j=0;
        }
        return i!=iEnd;
    }
};