Dare2Solve
The NestedIterator
problem involves flattening a nested list of integers. A nested list can contain integers or other lists, and the goal is to design an iterator that returns all integers from the nested structure in a sequential manner. The iterator should support two operations: next()
to get the next integer, and hasNext()
to check if there are more integers to iterate over.
The nested list structure can be represented as a tree where each integer is a leaf, and each list is a node that might contain other nodes or leaves. The key insight is to simulate a depth-first traversal of this tree, yielding integers whenever encountered. A stack or recursion can help in traversing the nested structure, keeping track of where we are in the nested lists.
Initialization: Start by pushing the iterator of the initial nested list onto a stack (for iterative solutions). Each item can either be an integer or another list.
hasNext()
: Check if the current element in the list is an integer or a nested list. If it’s an integer, return true
. If it’s a nested list, push the iterator for the nested list onto the stack and continue checking. Pop the stack when a sublist is exhausted.
next()
: Assume hasNext()
has been called, ensuring the iterator is at a valid integer. Retrieve and return the current integer and advance the iterator to the next position.
Recursive Approach (Python): The recursion keeps track of all nested lists by generating integers one by one, leveraging Python's yield
to flatten the structure.
The hasNext()
and next()
operations together visit each integer exactly once, thus the time complexity is proportional to the number of integers in the nested list, i.e., O(n), where n
is the total number of integers.
The space complexity depends on the depth of the nested lists. In the worst case, if all the lists are deeply nested, the space complexity would be O(d), where d
is the maximum depth of the nested list. In case of iterative solutions, it’s proportional to the depth of the list structure.
class NestedIterator {
stack<vector<NestedInteger>::iterator> begins, ends;
public:
NestedIterator(vector<NestedInteger> &nestedList) {
begins.push(nestedList.begin());
ends.push(nestedList.end());
}
int next() {
hasNext(); // Ensure the iterator is at a valid integer.
return (begins.top()++)->getInteger();
}
bool hasNext() {
while (!begins.empty()) {
if (begins.top() == ends.top()) { // Current list is fully traversed
begins.pop();
ends.pop();
} else if (begins.top()->isInteger()) {
return true;
} else { // The current element is a list
auto &nestedList = begins.top()->getList();
begins.top()++;
begins.push(nestedList.begin());
ends.push(nestedList.end());
}
}
return false;
}
};
class NestedIterator:
def __init__(self, nestedList: [NestedInteger]):
self.generator = self._flatten(nestedList)
self.nextVal = None
def _flatten(self, nestedList):
for item in nestedList:
if item.isInteger():
yield item.getInteger()
else:
yield from self._flatten(item.getList())
def next(self) -> int:
if not self.hasNext():
raise Exception("No more elements")
next_val = self.nextVal
self.nextVal = None
return next_val
def hasNext(self) -> bool:
if self.nextVal is not None:
return True
try:
self.nextVal = next(self.generator)
return True
except StopIteration:
return False
public class NestedIterator implements Iterator<Integer> {
private Stack<ListIterator<NestedInteger>> stack;
public NestedIterator(List<NestedInteger> nestedList) {
stack = new Stack<>();
stack.push(nestedList.listIterator());
}
@Override
public Integer next() {
hasNext(); // Ensure there's an integer ready to return.
return stack.peek().next().getInteger();
}
@Override
public boolean hasNext() {
while (!stack.isEmpty()) {
if (!stack.peek().hasNext()) {
stack.pop(); // Finished processing the current list.
} else {
NestedInteger x = stack.peek().next();
if (x.isInteger()) {
stack.peek().previous(); // Step back to handle it in `next()`
return true;
}
stack.push(x.getList().listIterator());
}
}
return false;
}
}
var NestedIterator = function (nestedList)
{
this.nestedList = nestedList;
this.iterator = flattenIteratorGen(this.nestedList);
this.currentValue = null;
};
/**
* @this NestedIterator
* @returns {boolean}
*/
NestedIterator.prototype.hasNext = function () {
if (this.currentValue !== null) {
return true;
}
const next = this.iterator.next();
if (next.done) {
return false;
}
this.currentValue = next.value;
return true;
};
/**
* @this NestedIterator
* @returns {number}
*/
NestedIterator.prototype.next = function () {
if (!this.hasNext()) {
throw new Error("No more elements to iterate.");
}
const value = this.currentValue;
this.currentValue = null;
return value;
};
const flattenIteratorGen = function* (nestedList) {
for (var item of nestedList) {
if (item.isInteger()) {
yield item.getInteger();
} else {
yield* flattenIteratorGen(item.getList());
}
}
};