## 341. Flatten Nested List Iterator

Given a nested list of integers, implement an iterator to flatten it.

Each element is either an integer, or a list -- whose elements may also be integers or other lists.

Example 1:

``````Input: [[1,1],2,[1,1]]
Output: [1,1,2,1,1]

Explanation: By calling next repeatedly until hasNext returns false,
the order of elements returned by nextshould be: [1,1,2,1,1].
``````

Example 2:

``````Input: [1,[4,[6]]]
Output: [1,4,6]

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

Thoughts:

1. Recursive structure: Use a stack to add element: from back to front
2. hasNext(): check the top element is integer, if not; flatten the top element and add it from back to front to the stack

Code

``````/**
* // This is the interface that allows for creating nested lists.
* // You should not implement it, or speculate about its implementation
* public interface NestedInteger {
*
*     // @return true if this NestedInteger holds a single integer, rather than a nested list.
*     public boolean isInteger();
*
*     // @return the single integer that this NestedInteger holds, if it holds a single integer
*     // Return null if this NestedInteger holds a nested list
*     public Integer getInteger();
*
*     // @return the nested list that this NestedInteger holds, if it holds a nested list
*     // Return null if this NestedInteger holds a single integer
*     public List<NestedInteger> getList();
* }
*/
public class NestedIterator implements Iterator<Integer> {
Stack<NestedInteger> stack;
public NestedIterator(List<NestedInteger> nestedList) {
stack = new Stack<>();
for(int i = nestedList.size() -1; i >= 0; i--){
stack.push(nestedList.get(i));
}
}

@Override
public Integer next() {
return stack.pop().getInteger();
}

@Override
public boolean hasNext() {
while(!stack.isEmpty()){
NestedInteger cur = stack.peek();
if(cur.isInteger()) return true;
stack.pop();
for(int i = cur.getList().size() - 1; i >=0; i--)
stack.push(cur.getList().get(i));
}
return false;
}
}

/**
* Your NestedIterator object will be instantiated and called as such:
* NestedIterator i = new NestedIterator(nestedList);
* while (i.hasNext()) v[f()] = i.next();
*/
``````