DFS+回溯专题13- leetcode341. Flatten Nested List Iterator

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341. Flatten Nested List Iterator

题目描述

给定一个嵌套的整数列表，用迭代器来flatten它。
每个元素要么是整数，要么是列表（同理，元素要么是整数，要么是其他列表）

例子

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].

思想
有点Trick的一道题…
(法1) 队列或栈来解决
(法2) 辅助空间，先把所有的整数存储起来（利用DFS）。

解法1

# """
# 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.nestedList = nestedList

    def next(self):
        """
        :rtype: int
        """
        if self.hasNext():
            return self.nestedList.pop(0).getInteger()

    def hasNext(self):
        """
        :rtype: bool
        """
        while self.nestedList:
            if self.nestedList[0].isInteger():
                return True
            self.nestedList = self.nestedList[0].getList() + self.nestedList[1:]
        return False

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

解法2

class NestedIterator(object):

    def __init__(self, nestedList):
        """
        Initialize your data structure here.
        :type nestedList: List[NestedInteger]
        """
        self.flatten = []
        self.dfs(nestedList)

    def dfs(self, nestedList):
        for x in nestedList:
            if x.isInteger():
                self.flatten.append(x.getInteger())
            else:
                self.dfs(x.getList())
        
    def next(self):
        """
        :rtype: int
        """
        return self.flatten.pop(0)

    def hasNext(self):
        """
        :rtype: bool
        """
        return self.flatten