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.
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]# """classNestedIterator(object):def__init__(self,nestedList):""" Initialize your data structure here. :type nestedList: List[NestedInteger] """ self.s = [[nestedList,0]]defnext(self):""" :rtype: int """ l, i = self.s[-1] self.s[-1][1] +=1return l[i].getInteger()defhasNext(self):""" :rtype: bool """# get to the point where we can find an integerwhile self.s: nl, i = self.s[-1]# the current list is exhaustedif i ==len(nl): self.s.pop()# current admidst an iteration through nonexhausted listelif nl[i].isInteger():returnTrue# otherwise next element must be a list# add this to the stack so we can backtrack to it laterelse: self.s[-1][1] +=1 self.s.append([nl[i].getList(), 0])# reached back to the root, iteration is completereturnFalse# Your NestedIterator object will be instantiated and called as such:# i, v = NestedIterator(nestedList), []# while i.hasNext(): v.append(i.next())
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).