# 341. Flatten Nested List Iterator

## Description

You are given a nested list of integers nestedList. Each element is either an integer or a list whose elements may also be integers or other lists. Implement an iterator to flatten it.

Implement the NestedIterator class:

• NestedIterator(List<NestedInteger> nestedList) Initializes the iterator with the nested list nestedList.
• int next() Returns the next integer in the nested list.
• boolean hasNext() Returns true if there are still some integers in the nested list and false otherwise.

Your code will be tested with the following pseudocode:

initialize iterator with nestedList
res = []
while iterator.hasNext()
append iterator.next() to the end of res
return res


If res matches the expected flattened list, then your code will be judged as correct.

Example 1:

Input: nestedList = [[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: nestedList = [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].


Constraints:

• 1 <= nestedList.length <= 500
• The values of the integers in the nested list is in the range [-106, 106].

## Solutions

2 possible approaches: DFS and iteration.

Iteration generally requires the use of the stack to assist traversal, due to the last-in first-out characteristic of the stack.

When we traverse the vector, push the object onto the stack from back to front, then the first object will be the first one to be taken out for processing after being pushed onto the stack.

The hasNext() function needs to traverse the stack and process it,

• If the top element of the stack is an integer, return true directly,
• If not, then remove the top element of the stack, and start traversing the retrieved list, or push it back to the stack

The loop stop condition is that the stack is empty and false is returned.

• /**
* // 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> {

private List<Integer> vals;

private Iterator<Integer> cur;

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

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

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

private void dfs(List<NestedInteger> nestedList) {
for (NestedInteger e : nestedList) {
if (e.isInteger()) {
} else {
dfs(e.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
* class NestedInteger {
*   public:
*     // Return true if this NestedInteger holds a single integer, rather than a nested list.
*     bool isInteger() const;
*
*     // 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
*     int getInteger() const;
*
*     // Return the nested list that this NestedInteger holds, if it holds a nested list
*     // The result is undefined if this NestedInteger holds a single integer
*     const vector<NestedInteger> &getList() const;
* };
*/

class NestedIterator {
public:
NestedIterator(vector<NestedInteger>& nestedList) {
dfs(nestedList);
}

int next() {
return vals[cur++];
}

bool hasNext() {
return cur < vals.size();
}

private:
vector<int> vals;
int cur = 0;

void dfs(vector<NestedInteger>& nestedList) {
for (auto& e : nestedList) {
if (e.isInteger()) {
vals.push_back(e.getInteger());
} else {
dfs(e.getList());
}
}
}
};

/**
* Your NestedIterator object will be instantiated and called as such:
* NestedIterator i(nestedList);
* while (i.hasNext()) cout << i.next();
*/

• # """
# This is the interface that allows for creating nested lists.
# You should not implement it, or speculate about its implementation
# """
# class NestedInteger:
#    def isInteger(self) -> bool:
#        """
#        @return True if this NestedInteger holds a single integer, rather than a nested list.
#        """
#
#    def getInteger(self) -> int:
#        """
#        @return the single integer that this NestedInteger holds, if it holds a single integer
#        Return None if this NestedInteger holds a nested list
#        """
#
#    def getList(self) -> [NestedInteger]:
#        """
#        @return the nested list that this NestedInteger holds, if it holds a nested list
#        Return None if this NestedInteger holds a single integer
#        """

from collections import deque

class NestedIterator:
def __init__(self, nestedList: [NestedInteger]):
self.stack = deque()
self.prepareStack(nestedList)

def next(self) -> int:
if not self.hasNext(): # trigger hasNext()
return None
self.hasNext()
return self.stack.pop().getInteger()

def hasNext(self) -> bool:
while self.stack and not self.stack[-1].isInteger():
# getList() more like get item, could be Integer or NestedInteger
lst = self.stack.pop().getList()
self.prepareStack(lst)
return bool(self.stack)

def prepareStack(self, nestedList):
for i in range(len(nestedList)-1, -1, -1):
self.stack.append(nestedList[i])

# Your NestedIterator object will be instantiated and called as such:
# i, v = NestedIterator(nestedList), []
# while i.hasNext(): v.append(i.next())

############

'''
>>> from collections import deque
>>>
>>> stack = deque()
>>> stack.append(3)
>>> stack.append(2)
>>> stack.append(1)
>>> stack
deque([3, 2, 1])
>>> stack.pop()
1
'''
class NestedIterator: # not working if memory is limited and input is huge list
def __init__(self, nestedList: [NestedInteger]):
def dfs(nestedList):
for e in nestedList:
if e.isInteger():
self.vals.append(e.getInteger())
else:
dfs(e.getList())

self.vals = []
dfs(nestedList)
self.cur = 0

def next(self) -> int:
res = self.vals[self.cur]
self.cur += 1
return res

def hasNext(self) -> bool:
return self.cur < len(self.vals)

# Your NestedIterator object will be instantiated and called as such:
# i, v = NestedIterator(nestedList), []
# while i.hasNext(): v.append(i.next())


• /**
* // 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 {
nested *list.List
}

func Constructor(nestedList []*NestedInteger) *NestedIterator {
nested := list.New()
for _, v := range nestedList {
nested.PushBack(v)
}
return &NestedIterator{nested: nested}
}

func (this *NestedIterator) Next() int {
res := this.nested.Front().Value.(*NestedInteger)
this.nested.Remove(this.nested.Front())
return res.GetInteger()
}

func (this *NestedIterator) HasNext() bool {
for this.nested.Len() > 0 && !this.nested.Front().Value.(*NestedInteger).IsInteger() {
front := this.nested.Front().Value.(*NestedInteger)
this.nested.Remove(this.nested.Front())
nodes := front.GetList()
for i := len(nodes) - 1; i >= 0; i-- {
this.nested.PushFront(nodes[i])
}
}
return this.nested.Len() > 0
}

• /**
* // This is the interface that allows for creating nested lists.
* // You should not implement it, or speculate about its implementation
* class NestedInteger {
*     If value is provided, then it holds a single integer
*     Otherwise it holds an empty nested list
*     constructor(value?: number) {
*         ...
*     };
*
*     Return true if this NestedInteger holds a single integer, rather than a nested list.
*     isInteger(): boolean {
*         ...
*     };
*
*     Return the single integer that this NestedInteger holds, if it holds a single integer
*     Return null if this NestedInteger holds a nested list
*     getInteger(): number | null {
*         ...
*     };
*
*     Set this NestedInteger to hold a single integer equal to value.
*     setInteger(value: number) {
*         ...
*     };
*
*     Set this NestedInteger to hold a nested list and adds a nested integer elem to it.
*         ...
*     };
*
*     Return the nested list that this NestedInteger holds,
*     or an empty list if this NestedInteger holds a single integer
*     getList(): NestedInteger[] {
*         ...
*     };
* };
*/

class NestedIterator {
private vals: number[];
private index: number;

constructor(nestedList: NestedInteger[]) {
this.index = 0;
this.vals = [];
this.dfs(nestedList);
}

dfs(nestedList: NestedInteger[]) {
for (const v of nestedList) {
if (v.isInteger()) {
this.vals.push(v.getInteger());
} else {
this.dfs(v.getList());
}
}
}

hasNext(): boolean {
return this.index < this.vals.length;
}

next(): number {
return this.vals[this.index++];
}
}

/**
* Your ParkingSystem object will be instantiated and called as such:
* var obj = new NestedIterator(nestedList)
* var a: number[] = []
* while (obj.hasNext()) a.push(obj.next());
*/


• // #[derive(Debug, PartialEq, Eq)]
// pub enum NestedInteger {
//   Int(i32),
//   List(Vec<NestedInteger>)
// }
struct NestedIterator {
index: usize,
vals: Vec<i32>,
}

/**
* &self means the method takes an immutable reference.
* If you need a mutable reference, change it to &mut self instead.
*/
impl NestedIterator {
fn dfs(nestedList: &Vec<NestedInteger>, vals: &mut Vec<i32>) {
for ele in nestedList.iter() {
match ele {
NestedInteger::Int(val) => vals.push(*val),
NestedInteger::List(list) => Self::dfs(list, vals),
}
}
}

fn new(nestedList: Vec<NestedInteger>) -> Self {
let mut vals = vec![];
Self::dfs(&nestedList, &mut vals);
Self {
vals,
index: 0,
}
}

fn next(&mut self) -> i32 {
let res = self.vals[self.index];
self.index += 1;
res
}

fn has_next(&self) -> bool {
self.index < self.vals.len()
}
}/**
* Your NestedIterator object will be instantiated and called as such:
* let obj = NestedIterator::new(nestedList);
* let ret_1: i32 = obj.next();
* let ret_2: bool = obj.has_next();
*/