341_FlattenNestedListIterator.py

#! /usr/bin/env python
# -*- coding: utf-8 -*-
# @Author: xuezaigds@gmail.com
# @Last Modified time: 2016-07-09 14:22:29

# """
# 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):
    """
    According to:
    https://discuss.leetcode.com/topic/42042/simple-java-solution-using-a-stack-with-explanation

    In the constructor, we push all the nestedList into the stack from back to front,
    so when we pop the stack, it returns the very first element.
    Second, in the hasNext() function, we peek the first element in stack currently,
    and if it is an Integer, we will return true and pop the element.

    If it is a list, we will further flatten it.
    This is iterative version of flatting the nested list.
    Again, we need to iterate from the back to front of the list.
    """
    def __init__(self, nestedList):
        """Initialize your data structure here.

        :type nestedList: List[NestedInteger]
        """
        self.stack = []
        for item in nestedList[::-1]:
            self.stack.append(item)

    def next(self):
        val = self.stack[-1].getInteger()
        self.stack.pop()
        return val

    def hasNext(self):
        while self.stack:
            curr = self.stack[-1]
            if curr.isInteger():
                return True
            self.stack.pop()
            if curr.getList():
                for i in curr.getList()[::-1]:
                    self.stack.append(i)
        return False


class NestedIterator_2(object):
    """ Python Generators solution

    According to:
    https://discuss.leetcode.com/topic/45844/python-generators-solution
    """
    def __init__(self, nestedList):
        """Initialize your data structure here.
        """
        def gen(nestedList):
            for item in nestedList:
                if item.isInteger():
                    yield item.getInteger()
                else:
                    for list in gen(item.getList()):
                        yield list

        self.gen = gen(nestedList)

    def next(self):
        return self.value

    def hasNext(self):
        try:
            self.value = next(self.gen)
            return True
        except StopIteration:
            return False

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

'''
[]
[1,[2,[3]]]
[[1,2],3,[4,5]]
[[[1,2,3], [4,5], 7], [8,9], 10]
'''