问题描述

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].

问题分析

运用栈的思想，当栈为空时，从NestedList头部拿一个元素到stack；
当stack的栈顶元素不为Integer，则取栈顶原素，将其列表成员按逆序加入栈；

需要注意的一点是，其实取next的主要工作是在hasNext做的，因为只有得到了next整数，才能证明有next，而取next只要取栈顶元素即可。还需要注意，NestedList可能是有空数组，例如“[[]]”。

AC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.nested = nestedList        

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

    def hasNext(self):
        """
        :rtype: bool
        """
        while self.stack or self.nested:
            if not self.stack:
                self.stack.append(self.nested.pop(0))
            while self.stack and not self.stack[-1].isInteger():
                tmp = self.stack.pop().getList()
                for t in tmp[::-1]:
                    self.stack.append(t)
            if self.stack:
                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())