# Question

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 next should 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].


# Solution

/**
* // 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> {
public NestedIterator(List<NestedInteger> nestedList) {
lists = new Stack<>();
lists.push(nestedList.listIterator());
verified = false;
}

public Integer next() {
if(!verified)
hasNext();
verified = false;
return lists.peek().next().getInteger();
}

public boolean hasNext() {
if(verified) return true;
while (!lists.empty()) {
if (!lists.peek().hasNext()) {
lists.pop();
} else {
NestedInteger x = lists.peek().next();
if (x.isInteger()) {
lists.peek().previous();
verified = true;
return true;
} else
lists.push(x.getList().listIterator());
}
}
return false;
}

private Stack<ListIterator<NestedInteger>> lists;
private boolean verified;
}

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