0341. Flatten Nested List Iterator

# 341. Flatten Nested List Iterator#

## 题目 #

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 callingnext repeatedly untilhasNext returns false,
the order of elements returned bynext should be:[1,1,2,1,1].
``````

Example 2:

``````Input:[1,[4,[6]]]
Output:[1,4,6]
Explanation:By callingnext repeatedly untilhasNext returns false,
the order of elements returned bynext should be:[1,4,6].

``````

## 解题思路 #

• 题目要求实现一个嵌套版的数组。可以用 `[]int` 实现，也可以用链表实现。笔者此处用链表实现。外层构造一个一维数组，一维数组内部每个元素是一个链表。额外还需要记录这个嵌套链表在原数组中的 `index` 索引。`Next()` 实现比较简单，取出对应的嵌套节点。`HasNext()` 方法则感觉嵌套节点里面的 `index` 信息判断是否还有 `next` 元素。

## 代码 #

``````/**
* // This is the interface that allows for creating nested lists.
* // You should not implement it, or speculate about its implementation
* type NestedInteger struct {
* }
*
* // Return true if this NestedInteger holds a single integer, rather than a nested list.
* func (this NestedInteger) IsInteger() bool {}
*
* // Return the single integer that this NestedInteger holds, if it holds a single integer
* // The result is undefined if this NestedInteger holds a nested list
* // So before calling this method, you should have a check
* func (this NestedInteger) GetInteger() int {}
*
* // Set this NestedInteger to hold a single integer.
* func (n *NestedInteger) SetInteger(value int) {}
*
* // Set this NestedInteger to hold a nested list and adds a nested integer to it.
* func (this *NestedInteger) Add(elem NestedInteger) {}
*
* // Return the nested list that this NestedInteger holds, if it holds a nested list
* // The list length is zero if this NestedInteger holds a single integer
* // You can access NestedInteger's List element directly if you want to modify it
* func (this NestedInteger) GetList() []*NestedInteger {}
*/

type NestedIterator struct {
stack *list.List
}

type listIndex struct {
nestedList []*NestedInteger
index int
}

func Constructor(nestedList []*NestedInteger) *NestedIterator {
stack := list.New()
stack.PushBack(&listIndex{nestedList, 0})
return &NestedIterator{stack}
}

func (this *NestedIterator) Next() int {
if !this.HasNext() {
return -1
}
last := this.stack.Back().Value.(*listIndex)
nestedList, i := last.nestedList, last.index
val := nestedList[i].GetInteger()
last.index++
return val
}

func (this *NestedIterator) HasNext() bool {
stack := this.stack
for stack.Len() > 0 {
last := stack.Back().Value.(*listIndex)
nestedList, i := last.nestedList, last.index
if i >= len(nestedList) {
stack.Remove(stack.Back())
} else {
val := nestedList[i]
if val.IsInteger() {
return true
}
last.index++
stack.PushBack(&listIndex{val.GetList(), 0})
}
}
return false
}
``````

Apr 8, 2023