[LeetCode] 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:
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].

代码如下：

import java.util.Iterator;import java.util.List;import java.util.ListIterator;import java.util.Stack;      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;Stack<ListIterator<NestedInteger>> stack=new Stack<ListIterator<NestedInteger>>();    public NestedIterator(List<NestedInteger> nestedList) {     list=nestedList;         stack.push(list.listIterator());    }    @Override    public Integer next() {        if(!hasNext()) return null;        return stack.peek().next().getInteger();    }    @Override    public boolean hasNext() {    if(stack.isEmpty()) return false;    while(!stack.isEmpty()){    if(!stack.peek().hasNext()){    stack.pop(); continue;    }    NestedInteger integer=stack.peek().next();    if(integer.isInteger()){    stack.peek().previous();    return true;    }else{    stack.push(integer.getList().listIterator());    }    }    return false;    }}/** * Your NestedIterator object will be instantiated and called as such: * NestedIterator i = new NestedIterator(nestedList); * while (i.hasNext()) v[f()] = i.next(); */