Flatten Nested List Iterator

要点：题的本质是建立next和isInteger/getList/getInteger的关系，考点和tree的遍历类似：用stack存dfs的trace，这样可以backtrack到之前层的结点。

• 注意NestedInteger类的表示，清楚了表示很多题就迎刃而解了：NestedInteger表示一个结点：既可以是List也可以是Integer。什么类型由interface的get method返回值决定。而最外层object是List，这样表示不可能以Integer为起点
• 为什么需要stack？本质上是tree的结构：遍历的时候如果当前元素是list，那么之后要，向更深遍历，并且要能返回的上层继续下一个元素。
• 如何unwind？对于list，相当于子树展开，这里就把list中元素反向push到stack里。因为list可能为空，这样就没有下一个元素可以iterate，所以要多层展开（也就是说，stack top是不能为list的）直到一个integer元素或者没有其他可遍历元素为止。因此，在init和next得到当前元素后，要用prepare来展开stack top的可能list

# """# 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.stk = []        for i in xrange(len(nestedList)-1, -1, -1):            self.stk.append(nestedList[i])        self._prepare()            def _prepare(self):        while self.stk and not self.stk[-1].isInteger():            topList = self.stk.pop().getList()            for i in xrange(len(topList)-1, -1, -1):                self.stk.append(topList[i])    def next(self):        """        :rtype: int        """        topInt = self.stk.pop().getInteger()        self._prepare()        return topInt    def hasNext(self):        """        :rtype: bool        """        return self.stk        # Your NestedIterator object will be instantiated and called as such:# i, v = NestedIterator(nestedList), []# while i.hasNext(): v.append(i.next())