Peeking Iterator Design an iterator that supports the peek
operation on an existing iterator in addition to the hasNext
and the next
operations.
Implement the PeekingIterator
class:
PeekingIterator(Iterator<int> nums)
Initializes the object with the given integer iteratoriterator
.int next()
Returns the next element in the array and moves the pointer to the next element.boolean hasNext()
Returnstrue
if there are still elements in the array.int peek()
Returns the next element in the array without moving the pointer.
Note: Each language may have a different implementation of the constructor and Iterator
, but they all support the int next()
and boolean hasNext()
functions.
Example 1:
Input ["PeekingIterator", "next", "peek", "next", "next", "hasNext"] [[[1, 2, 3]], [], [], [], [], []] Output[null, 1, 2, 2, 3, false]
Explanation PeekingIterator peekingIterator = new PeekingIterator([1, 2, 3]); // [1,2,3] peekingIterator.next(); // return 1, the pointer moves to the next element [1,2,3]. peekingIterator.peek(); // return 2, the pointer does not move [1,2,3]. peekingIterator.next(); // return 2, the pointer moves to the next element [1,2,3] peekingIterator.next(); // return 3, the pointer moves to the next element [1,2,3] peekingIterator.hasNext(); // return False
Constraints:
1 <= nums.length <= 1000
1 <= nums[i] <= 1000
- All the calls to
next
andpeek
are valid. - At most
1000
calls will be made tonext
,hasNext
, andpeek
.
Peeking Iterator Solutions
✅Time: O(1)
✅Space: O(n)
C++
class PeekingIterator : public Iterator {
public:
PeekingIterator(const vector<int>& nums) : Iterator(nums) {}
// Returns the next element in the iteration without advancing the iterator.
int peek() {
// Iterator(*this) makes a copy of current iterator, then call next on the
// copied iterator to get the next value without affecting current iterator
return Iterator(*this).next();
}
// hasNext() and next() should behave the same as in the Iterator interface.
// Override them if needed.
int next() {
return Iterator::next();
}
bool hasNext() const {
return Iterator::hasNext();
}
};
Java
// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
class PeekingIterator implements Iterator<Integer> {
public PeekingIterator(Iterator<Integer> iterator) {
this.iterator = iterator;
buffer = iterator.hasNext() ? iterator.next() : null;
}
// Returns the next element in the iteration without advancing the iterator.
public Integer peek() {
return buffer;
}
// hasNext() and next() should behave the same as in the Iterator interface.
// Override them if needed.
@Override
public Integer next() {
Integer next = buffer;
buffer = iterator.hasNext() ? iterator.next() : null;
return next;
}
@Override
public boolean hasNext() {
return buffer != null;
}
private Iterator<Integer> iterator;
private Integer buffer;
}
Python
class PeekingIterator:
def __init__(self, iterator: Iterator):
self.iterator = iterator
self.buffer = self.iterator.next() if self.iterator.hasNext() else None
def peek(self) -> int:
"""
Returns the next element in the iteration without advancing the iterator.
"""
return self.buffer
def next(self) -> int:
next = self.buffer
self.buffer = self.iterator.next() if self.iterator.hasNext() else None
return next
def hasNext(self) -> bool:
return self.buffer is not None
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