给你一个嵌套的整数列表 nestedList 。每个元素要么是一个整数，要么是一个列表；该列表的元素也可能是整数或者是其他列表。请你实现一个迭代器将其扁平化，使之能够遍历这个列表中的所有整数。

实现扁平迭代器类 NestedIterator ：

NestedIterator(List nestedList) 用嵌套列表 nestedList 初始化迭代器。
int next() 返回嵌套列表的下一个整数。
boolean hasNext() 如果仍然存在待迭代的整数，返回 true ；否则，返回 false 。
你的代码将会用下述伪代码检测：

initialize iterator with nestedList
res = []
while iterator.hasNext()
append iterator.next() to the end of res
return res
如果 res 与预期的扁平化列表匹配，那么你的代码将会被判为正确。

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/flatten-nested-list-iterator
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

递归

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;


public class NestedIterator implements Iterator<Integer> {

    private List<Integer> realList;

    private Iterator<Integer> cur;

    public NestedIterator(List<NestedInteger> nestedList) {
        this.realList = new ArrayList<>();
        dfs(nestedList);
        cur = realList.iterator();
    }

    @Override
    public Integer next() {
        return cur.next();
    }

    @Override
    public boolean hasNext() {
        return cur.hasNext();
    }

    private void dfs(List<NestedInteger> nestedList) {
        for (NestedInteger nestedInteger : nestedList) {
            if (nestedInteger.isInteger()) {
                realList.add(nestedInteger.getInteger());
            } else {
                dfs(nestedInteger.getList());
            }
        }
    }
}

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

// This is the interface that allows for creating nested lists.
// You should not implement it, or speculate about its implementation
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 empty list if this NestedInteger holds a single integer
    public List<NestedInteger> getList();
}

迭代

import java.util.*;

public class NestedIterator implements Iterator<Integer> {

    private Deque<Iterator<NestedInteger>> stack;

    public NestedIterator(List<NestedInteger> nestedList) {
        stack = new LinkedList<>();
        stack.push(nestedList.iterator());
    }

    @Override
    public Integer next() {
        return stack.peek().next().getInteger();
    }

    @Override
    public boolean hasNext() {
        while (!stack.isEmpty()) {
            Iterator<NestedInteger> integerIterator = stack.peek();
            if (!integerIterator.hasNext()) {
                stack.pop();
                continue;
            }
            NestedInteger next = integerIterator.next();
            if (next.isInteger()) {
                stack.push(new ArrayList<NestedInteger>() {{
                    add(next);
                }}.iterator());
                return true;
            }
            stack.push(next.getList().iterator());
        }
        return false;
    }
}

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

// This is the interface that allows for creating nested lists.
// You should not implement it, or speculate about its implementation
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 empty list if this NestedInteger holds a single integer
    public List<NestedInteger> getList();
}