341. Flatten Nested List Iterator

Problem Statement

link: LeetCode.cn LeetCode 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 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:

Input: nestedList = [[1,1],2,[1,1]] Output: [1,1,2,1,1]

Input: nestedList = [1,[4,[6]]] Output: [1,4,6]

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

Solution Approach

The solution involves using a stack to iteratively flatten the nested list, ensuring that all nested lists are fully expanded before retrieving the next integer.

Algorithm

1. Initialize a stack with the reversed nested list to maintain the order of elements for processing.
2. Iteratively check the top element of the stack: if it’s a list, expand it by adding its elements back to the stack in reverse order; if it’s an integer, keep it ready for retrieval.
3. Pop and return the integer from the stack when next() is called, and continue flattening the nested lists until all elements are processed.

Implement

    class NestedIterator:
    def __init__(self, nestedList: [NestedInteger]):
        self.stack = nestedList[::-1]
    
    def next(self) -> int:
        return self.stack.pop(-1).getInteger()
    
    def hasNext(self) -> bool:
        while len(self.stack) > 0 and self.stack[-1].isInteger() is False:
            self.stack += self.stack.pop().getList()[::-1]
        return len(self.stack) > 0

    
    

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