题目描述：

LeetCode 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].

题目大意：

给定一个嵌套的整数列表，实现一个迭代器将其展开。

每一个元素或者是一个整数，或者是一个列表 -- 其元素也是一个整数或者其他列表。

测试用例如题目描述。

解题思路：

利用栈（Stack）数据结构对嵌套列表展开，在hasNext方法内将下一个需要访问的整数元素准备好，详见代码。

Python代码：

# """
# 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.stack = []
        self.list = nestedList

    def next(self):
        """
        :rtype: int
        """
        return self.stack.pop()
            
    def hasNext(self):
        """
        :rtype: bool
        """
        while self.list or self.stack:
            if not self.stack:
                self.stack.append(self.list.pop(0))
            while self.stack and not self.stack[-1].isInteger():
                top = self.stack.pop().getList()
                for e in top[::-1]:
                    self.stack.append(e)
            if self.stack and self.stack[-1].isInteger():
                return True
        return False

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