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:

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].

解题要点：

用stack的特性，来倒叙装元素，保持最新的在最顶层。然后再hasNext里检测元素是否为int，如是返回true；如不是，把list里的元素倒叙再放到stack顶层。

# """
# 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 = []
        for i in range(len(nestedList)-1, -1, -1):
            self.stack.append(nestedList[i])
        # print self.stack

    def next(self):
        """
        :rtype: int
        """
        return self.stack.pop().getInteger()

    def hasNext(self):
        """
        :rtype: bool
        """
        while self.stack:
            if self.stack[-1].isInteger():
                return True
            p = self.stack.pop()
            for i in range(len(p.getList())-1, -1, -1):
                self.stack.append(p.getList()[i])
        return False

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