边工作边刷题：70天一遍leetcode: day 45

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())