Problem 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 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.
Key Insights
- The problem requires a mechanism to traverse a potentially deeply nested structure.
- A stack can be used to keep track of the current position in the nested list for efficient traversal.
- The iterator must be able to return integers in the correct order, flattening the nested structure.
Space and Time Complexity
Time Complexity: O(N), where N is the total number of integers in the nested list. Space Complexity: O(D), where D is the maximum depth of the nested list, due to the stack used for traversal.
Solution
To solve this problem, we can use a stack-based approach. The main idea is to use a stack to keep track of the elements we need to process. When calling hasNext(), we will ensure that the top of the stack contains an integer. If the top of the stack is a list, we will pop it and push all its elements onto the stack. This way, we can keep flattening the nested structure until we reach an integer. The next() method will then simply pop and return the integer from the stack.