# 341 Flatten Nested List Iterator

### Problem:

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: Given the list [[1,1],2,[1,1]],

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

Example 2: Given the list [1,[4,[6]]],

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

### Solutions:

``````/**
* // 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> {

private Stack<Iterator<NestedInteger>> s = new Stack<Iterator<NestedInteger>>();
private Integer next = null;
public NestedIterator(List<NestedInteger> nestedList) {
Iterator<NestedInteger> it = nestedList.iterator();
s.push(it);
findNext();
}
private void findNext() {
next = null;
Iterator<NestedInteger> it = null;
while (!s.isEmpty()) {
it = s.peek();
if (!it.hasNext()) {
s.pop();
continue;
}
NestedInteger ni = it.next();
if (ni.isInteger()) {
next = ni.getInteger();
break;
}
else {
s.push(ni.getList().iterator());
}
}
}
@Override
public Integer next() {
Integer result = next;
findNext();
return result;
}

@Override
public boolean hasNext() {
return next != null;
}
}

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