/**

* // 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<Iterator<NestedInteger>> stk;

Iterator<NestedInteger> iter;

Integer num = null;

public NestedIterator(List<NestedInteger> nestedList) {

stk = new Stack<Iterator<NestedInteger>>();

iter = nestedList.iterator();

}

@Override

public Integer next() {

Integer ans = null;

if (num!=null) {//just runned hasNext()

ans = num;

num = null;

return ans;

}else{

if (hasNext()) {

ans = num;

num = null;

return ans;

}else{

return null;

}

}

}

// [[1],2]

@Override

public boolean hasNext() {

NestedInteger temp = null;

//push stage

while(true){

while(iter.hasNext()){

if ((temp = iter.next()).isInteger()) {

num = temp.getInteger();

return true;

}else{

stk.push(iter);

iter = temp.getList().iterator();

}

}

//pop stage

while(!iter.hasNext()){

if (stk.empty()) {

return false;

}else{

iter = stk.pop();

}

}

}

}

}

/**

* Your NestedIterator object will be instantiated and called as such:

* NestedIterator i = new NestedIterator(nestedList);

* while (i.hasNext()) v[f()] = i.next();

*/