Flatten Nested List Iterator
Created: November 13, 2018 by [lek-tin]
Last updated: March 14, 2020
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 calling next repeatedly until hasNext returns false, the order of elements returned by next should be: `[1,1,2,1,1]`.
Example 2
Input: [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]`.
Solution (preprocessing recursively)
# """
# 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]
"""
arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
print(arr[::-1])
self.flat = []
def flatten(nested):
for n in nested:
if n.isInteger():
self.flat.append(n.getInteger())
else:
flatten(n.getList())
flatten(nestedList)
self.flat = self.flat[::-1]
def next(self):
"""
:rtype: int
"""
return self.flat.pop()
def hasNext(self):
"""
:rtype: bool
"""
# equivalent to len(self.flat) > 0
return bool(self.flat)
# Your NestedIterator object will be instantiated and called as such:
# i, v = NestedIterator(nestedList), []
# while i.hasNext(): v.append(i.next())
Solution (stack)
# """
# 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
# """
class NestedIterator(object):
def __init__(self, nestedList):
"""
Initialize your data structure here.
:type nestedList: List[NestedInteger]
"""
self.stack = []
self.flatten(nestedList)
def next(self):
"""
:rtype: int
"""
return self.stack.pop().getInteger() if self.hasNext() else None
def hasNext(self):
"""
:rtype: bool
"""
while self.stack:
if self.stack[-1].isInteger():
return True
else:
# a nestedList need to be flatten and its root-level items need to be placed on top of the stack
tempList = self.stack.pop().getList()
self.flatten(tempList)
return False
def flatten(self, nestedList):
for i in range(len(nestedList)-1, -1, -1):
# push each NestedInteger into stack, it may be an integer or nestedList
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())
Solution (stack + iterator)
# """
# 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
# """
class NestedIterator(object):
def __init__(self, nestedList):
"""
Initialize your data structure here.
:type nestedList: List[NestedInteger]
"""
self.stack = [iter(nestedList)]
self.curr = None
def next(self):
"""
:rtype: int
"""
result = self.curr
self.curr = None
return result
def hasNext(self):
"""
:rtype: bool
"""
while self.stack and self.curr == None:
top = self.stack[-1]
top_next = next(top, None)
if top_next == None:
self.stack.pop()
continue
else:
if top_next.isInteger():
self.curr = top_next.getInteger()
return True
else:
self.stack.append( iter(top_next.getList()) )
return False
# Your NestedIterator object will be instantiated and called as such:
# i, v = NestedIterator(nestedList), []
# while i.hasNext(): v.append(i.next())