341. 扁平化嵌套列表迭代器

题目

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<NestedInteger> 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。

示例：

输入：nestedList = [[1,1],2,[1,1]]
输出：[1,1,2,1,1]
解释：通过重复调用 next 直到 hasNext 返回 false，next 返回的元素的顺序应该是: [1,1,2,1,1]。

https://leetcode.cn/problems/flatten-nested-list-iterator/

解法

/**
 * // 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 empty list if this NestedInteger holds a single integer
 *     public List<NestedInteger> getList();
 * }
 */
public class NestedIterator implements Iterator<Integer> {

    private Deque<NestedInteger> stack = new ArrayDeque<>();

    public NestedIterator(List<NestedInteger> nestedList) {
        for(int i = nestedList.size() -1; i >= 0; i--) {
            NestedInteger lastElement = nestedList.get(i);
            stack.offerFirst(lastElement);
        }
    }

    @Override
    public Integer next() {
        if (hasNext()) {
            return stack.pollFirst().getInteger();
        }
        throw new RuntimeException("请先调研HasNext()做判断");
    }

    @Override
    public boolean hasNext() {
        if (stack.isEmpty()) {
            return false;
        } else {
            if (stack.peekFirst().isInteger()) {
                //此时元素是Integer
                return true;
            } else {
                //此时元素是个List
                List<NestedInteger> list = stack.pollFirst().getList();
                for(int i = list.size() -1; i >= 0; i --) {
                    NestedInteger lastElement = list.get(i);
                    stack.offerFirst(lastElement);
                }
                return hasNext();
            }
        }
    }
}

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