341-flatten-nested-list-iterator

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Description

You are given a nested list of integers nestedList. Each element is either an integer or a list whose elements may also be integers or other lists. Implement an iterator to flatten it.

Implement the NestedIterator class:

• NestedIterator(List<NestedInteger> nestedList) Initializes the iterator with the nested list nestedList.
• int next() Returns the next integer in the nested list.
• boolean hasNext() Returns true if there are still some integers in the nested list and false otherwise.

Your code will be tested with the following pseudocode:

initialize iterator with nestedList
res = []
while iterator.hasNext()
    append iterator.next() to the end of res
return res

If res matches the expected flattened list, then your code will be judged as correct.

 

Example 1:

Input: nestedList = [[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: nestedList = [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].

 

Constraints:

• 1 <= nestedList.length <= 500
• The values of the integers in the nested list is in the range [-106, 106].

Solution(Python)

# """
# This is the interface that allows for creating nested lists.
# You should not implement it, or speculate about its implementation
# """
# class NestedInteger:
#    def isInteger(self) -> bool:
#        """
#        @return True if this NestedInteger holds a single integer, rather than a nested list.
#        """
#
#    def getInteger(self) -> int:
#        """
#        @return the single integer that this NestedInteger holds, if it holds a single integer
#        Return None if this NestedInteger holds a nested list
#        """
#
#    def getList(self) -> [NestedInteger]:
#        """
#        @return the nested list that this NestedInteger holds, if it holds a nested list
#        Return None if this NestedInteger holds a single integer
#        """


class NestedIterator:
    def __init__(self, nestedList: [NestedInteger]):
        self.itr = OptimalNestedIterator(nestedList)

    def next(self) -> int:
        return self.itr.next()

    def hasNext(self) -> bool:
        return self.itr.hasNext()


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


class NaiveNestedIterator:
    def __init__(self, nestedList: [NestedInteger]):
        self.arr = deque(self.chain(nestedList))

    def chain(self, cur: [NestedInteger]):
        tmp = []
        for val in cur:
            if val.isInteger():
                tmp.append(val.getInteger())
            else:
                tmp.extend(self.chain(val.getList()))
        return tmp

    def next(self) -> int:
        return self.arr.popleft()

    def hasNext(self) -> bool:
        return self.arr


class OptimalNestedIterator:
    def __init__(self, nestedList):
        def gen(nestedList):
            for x in nestedList:
                if x.isInteger():
                    yield x.getInteger()
                else:
                    for y in gen(x.getList()):
                        yield y

        self.gen = gen(nestedList)

    def next(self):
        return self.value

    def hasNext(self):
        try:
            self.value = next(self.gen)
            return True
        except StopIteration:
            return False