# 341 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]`.
The Idea: A nestedList is nothing but a tree. Naturally, a tree is recursively traversed, but we don't have that luxiery here because call stack has to be saved, in a sense. Any nestedList can be represented as a cons cell (lisp idea). For example, the tree below represents the following list: `(1 (2 6 7 8) 3 (4 (9 12)) (5 10 11))`. As we can see, following a preorder traversal through this tree will reveal the correct order of the elements within the nested list. A preorder traversal can be accomplished iteratively using a stack.
The approach to this problem is the same, except that we'll need to store an iterator for every sublist.
Any element within a nestedList can be either of two things: an integer or a list. If the element is an integer, we're ok to output it. Otherwise, if it is a list, we need to push this list onto the stack, and initialize it's own iterator (starting from 0).
Complexity: O(N) time where N is the total number of element in the list (including the nested lists), and O(|deepest nestest list|) space
# """
# This is the interface that allows for creating nested lists.
# You should not implement it, or speculate about its implementation
# """
# class NestedInteger(object):
# def isInteger(self):
# """
# @return True if this NestedInteger holds a single integer, rather than a nested list.
# :rtype bool
# """
#
# def getInteger(self):
# """
# @return the single integer that this NestedInteger holds, if it holds a single integer
# Return None if this NestedInteger holds a nested list
# :rtype int
# """
#
# def getList(self):
# """
# @return the nested list that this NestedInteger holds, if it holds a nested list
# Return None if this NestedInteger holds a single integer
# :rtype List[NestedInteger]
# """
class NestedIterator(object):
def __init__(self, nestedList):
"""
Initialize your data structure here.
:type nestedList: List[NestedInteger]
"""
self.s = [[nestedList, 0]]
def next(self):
"""
:rtype: int
"""
l, i = self.s[-1]
self.s[-1][1] += 1
return l[i].getInteger()
def hasNext(self):
"""
:rtype: bool
"""
# get to the point where we can find an integer
while self.s:
nl, i = self.s[-1]
# the current list is exhausted
if i == len(nl):
self.s.pop()
# current admidst an iteration through nonexhausted list
elif nl[i].isInteger():
return True
# otherwise next element must be a list
# add this to the stack so we can backtrack to it later
else:
self.s[-1][1] += 1
self.s.append([nl[i].getList(), 0])
# reached back to the root, iteration is complete
return False
# Your NestedIterator object will be instantiated and called as such:
# i, v = NestedIterator(nestedList), []
# while i.hasNext(): v.append(i.next())