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 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].

/**
 * // 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> {
    
    
    List<NestedInteger> list = null;
    Stack<NestedInteger> stack = null;
    public NestedIterator(List<NestedInteger> nestedList) {
        this.list = nestedList;
        this.stack = new Stack<>();
        for(int i = list.size()-1;i>=0;i--){
            stack.push(list.get(i));
        }
    }

    @Override
    public Integer next() {
        
        if(hasNext()){
            return stack.pop().getInteger();
        }
        
        
        return -1;
    }

    @Override
    public boolean hasNext() {
        
//         if(stack.isEmpty())
//             return false;
        
//         if(stack.top().isInteger())
//             return true;
        
//         List<NestedInteger> tmpList = stack.pop().getList();
        
//         for(int i = tmpList.size()-1;i>=0;i--){
//             stack.push(tmpList.get(i));
//         }
        
        while(!stack.isEmpty()){
            NestedInteger ni = stack.peek();
            if(ni.isInteger())
                return true;
            stack.pop();
            
            List<NestedInteger> list = ni.getList();
            
            for(int i = list.size()-1;i >= 0; i--){
                stack.push(list.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();
 */

use list iterator

public class NestedIterator implements Iterator<Integer> {

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

    public Integer next() {
        hasNext();
        return lists.peek().next().getInteger();
    }

    public boolean hasNext() {
        while (!lists.empty()) {
            if (!lists.peek().hasNext()) {
                lists.pop();
            } else {
                NestedInteger x = lists.peek().next();
                if (x.isInteger())
                
                    //也可以写成：
                    //stack.peek().previous()
                    //return true
                    //要previous的原因是在hasNext这一步，探测到了x是integer，但是
                    //这个时候，iterator已经pass了x，必须再往前走一步，不然next里get不到
                    return lists.peek().previous() == x;
                lists.push(x.getList().listIterator());
            }
        }
        return false;
    }
    
    private Stack<ListIterator<NestedInteger>> lists;
}