Flatten 2D Vector

Implement a iterator to flatten a 2d vector.

For example,
Given 2d vector =

[  [up],  [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:

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

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

Solution One: Without iterator

 Public classVector2dImplementsIterator<integer> {        intindexlist; intIndexele; List<List<Integer>>VEC;  PublicVector2d (list<list<integer>>vec2d) {Indexlist= 0; Indexele= 0; VEC=vec2d; } @Override PublicInteger Next () {Hasnext (); returnVec.get (indexlist). Get (indexele++); } @Override Public BooleanHasnext () { while(indexlist<vec.size ()) {            if(Indexele<vec.get (indexlist). Size ())return true; Else{indexlist++; Indexele=0; }        }        return false; }            }/*** Your vector2d object would be instantiated and called as such: * vector2d i = new vector2d (vec2d), * while (I.hasnex T ()) v[f ()] = I.next (); */

Solution two: With iterator

 Public classVector2dImplementsIterator<integer> {    PrivateIterator<list<integer>>i; PrivateIterator<integer>J;  PublicVector2d (list<list<integer>>vec2d) {i=Vec2d.iterator (); }     PublicInteger Next () {Hasnext (); returnJ.next (); }     Public BooleanHasnext () { while(J = =NULL|| !j.hasnext ()) &&I.hasnext ()) J=I.next (). iterator (); returnJ! =NULL&&J.hasnext (); }}/*** Your vector2d object would be instantiated and called as such: * vector2d i = new vector2d (vec2d), * while (I.hasnex T ()) v[f ()] = I.next (); */

Reference

Https://discuss.leetcode.com/topic/20697/7-9-lines-added-java-and-c-o-1-space/2

Flatten 2D Vector