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Question
Formatted question description: https://leetcode.ca/all/284.html
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
.
Follow up: How would you extend your design to be generic and work with all types, not just integer?
Algorithm
You can define a variable preFetch
to save the next value, use a bool variable to mark whether the next value is saved, and then call some of the original member functions.
Code
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import java.util.Iterator; class PeekingIterator implements Iterator<Integer> { private Integer preFetch = null; private Iterator<Integer> iterator; public PeekingIterator(Iterator<Integer> iterator) { // initialize any member here. this.iterator = iterator; if (this.iterator.hasNext()) preFetch = this.iterator.next(); } // Returns the preFetch element in the iteration without advancing the iterator. public Integer peek() { return preFetch; } // hasNext() and preFetch() should behave the same as in the Iterator interface. // Override them if needed. @Override public Integer next() { Integer toReturn = preFetch; preFetch = iterator.hasNext() ? iterator.next() : null; return toReturn; } @Override public boolean hasNext() { return preFetch != null; } } ///////////////////////////////// // I used 2 flags to indicate if it's end of an iterator // but, Integer can be noll, but int cannot be null. So, set preFetch to be null will solve the problem public class Peeking_Iterator { // Java Iterator interface reference: // https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html class PeekingIterator implements Iterator<Integer> { int preFetch; Iterator<Integer> iterator; boolean isIteratorEmpty; boolean isLastPrefetchPopped; public PeekingIterator(Iterator<Integer> iterator) { this.iterator = iterator; // initialize any member here. if (this.iterator.hasNext()) { preFetch = this.iterator.next(); } else { isIteratorEmpty = true; isLastPrefetchPopped = true; } } // Returns the preFetch element in the iteration without advancing the iterator. public Integer peek() { if (isIteratorEmpty && isLastPrefetchPopped) { // cannot peek, throw some exception here return null; } else { return preFetch; } } // hasNext() and preFetch() should behave the same as in the Iterator interface. // Override them if needed. @Override public Integer next() { if (isIteratorEmpty && isLastPrefetchPopped) { // cannot peek, throw some exception here return null; } int toReturn = preFetch; if (this.iterator.hasNext()) { preFetch = this.iterator.next(); } else { isIteratorEmpty = true; isLastPrefetchPopped= true; } return preFetch; } @Override public boolean hasNext() { if (isIteratorEmpty && isLastPrefetchPopped) { return false; } else { return true; } } } } ############ // Java Iterator interface reference: // https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html class PeekingIterator implements Iterator<Integer> { private Iterator<Integer> iterator; private boolean hasPeeked; private Integer peekedElement; public PeekingIterator(Iterator<Integer> iterator) { // initialize any member here. this.iterator = iterator; } // Returns the next element in the iteration without advancing the iterator. public Integer peek() { if (!hasPeeked) { peekedElement = iterator.next(); hasPeeked = true; } return peekedElement; } // hasNext() and next() should behave the same as in the Iterator interface. // Override them if needed. @Override public Integer next() { if (!hasPeeked) { return iterator.next(); } Integer result = peekedElement; hasPeeked = false; peekedElement = null; return result; } @Override public boolean hasNext() { return hasPeeked || iterator.hasNext(); } }
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# Below is the interface for Iterator, which is already defined for you. # # class Iterator: # def __init__(self, nums): # """ # Initializes an iterator object to the beginning of a list. # :type nums: List[int] # """ # # def hasNext(self): # """ # Returns true if the iteration has more elements. # :rtype: bool # """ # # def next(self): # """ # Returns the next element in the iteration. # :rtype: int # """ class PeekingIterator: def __init__(self, iterator): """ Initialize your data structure here. :type iterator: Iterator """ self.iterator = iterator self.has_peeked = False self.peeked_element = None def peek(self): """ Returns the next element in the iteration without advancing the iterator. :rtype: int """ if not self.has_peeked: self.peeked_element = self.iterator.next() self.has_peeked = True return self.peeked_element def next(self): """ :rtype: int """ if not self.has_peeked: return self.iterator.next() result = self.peeked_element self.has_peeked = False self.peeked_element = None return result def hasNext(self): """ :rtype: bool """ return self.has_peeked or self.iterator.hasNext() # Your PeekingIterator object will be instantiated and called as such: # iter = PeekingIterator(Iterator(nums)) # while iter.hasNext(): # val = iter.peek() # Get the next element but not advance the iterator. # iter.next() # Should return the same value as [val]. ############ # Below is the interface for Iterator, which is already defined for you. # # class Iterator(object): # def __init__(self, nums): # """ # Initializes an iterator object to the beginning of a list. # :type nums: List[int] # """ # # def hasNext(self): # """ # Returns true if the iteration has more elements. # :rtype: bool # """ # # def next(self): # """ # Returns the next element in the iteration. # :rtype: int # """ class PeekingIterator(object): def __init__(self, iterator): """ Initialize your data structure here. :type iterator: Iterator """ self.iter = iterator self.nextElem = None def peek(self): """ Returns the next element in the iteration without advancing the iterator. :rtype: int """ if self.nextElem: return self.nextElem if self.iter.hasNext(): self.nextElem = self.iter.next() return self.nextElem def next(self): """ :rtype: int """ ret = self.nextElem if self.nextElem: self.nextElem = None return ret return self.iter.next() def hasNext(self): """ :rtype: bool """ return (self.nextElem is not None) or self.iter.hasNext() # Your PeekingIterator object will be instantiated and called as such: # iter = PeekingIterator(Iterator(nums)) # while iter.hasNext(): # val = iter.peek() # Get the next element but not advance the iterator. # iter.next() # Should return the same value as [val].
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/* * Below is the interface for Iterator, which is already defined for you. * **DO NOT** modify the interface for Iterator. * * class Iterator { * struct Data; * Data* data; * public: * Iterator(const vector<int>& nums); * Iterator(const Iterator& iter); * * // Returns the next element in the iteration. * int next(); * * // Returns true if the iteration has more elements. * bool hasNext() const; * }; */ class PeekingIterator : public Iterator { public: PeekingIterator(const vector<int>& nums) : Iterator(nums) { // Initialize any member here. // **DO NOT** save a copy of nums and manipulate it directly. // You should only use the Iterator interface methods. hasPeeked = false; } // Returns the next element in the iteration without advancing the iterator. int peek() { if (!hasPeeked) { peekedElement = Iterator::next(); hasPeeked = true; } return peekedElement; } // hasNext() and next() should behave the same as in the Iterator interface. // Override them if needed. int next() { if (!hasPeeked) return Iterator::next(); hasPeeked = false; return peekedElement; } bool hasNext() const { return hasPeeked || Iterator::hasNext(); } private: bool hasPeeked; int peekedElement; };
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/* Below is the interface for Iterator, which is already defined for you. * * type Iterator struct { * * } * * func (this *Iterator) hasNext() bool { * // Returns true if the iteration has more elements. * } * * func (this *Iterator) next() int { * // Returns the next element in the iteration. * } */ type PeekingIterator struct { iter *Iterator hasPeeked bool peekedElement int } func Constructor(iter *Iterator) *PeekingIterator { return &PeekingIterator{iter, iter.hasNext(), iter.next()} } func (this *PeekingIterator) hasNext() bool { return this.hasPeeked || this.iter.hasNext() } func (this *PeekingIterator) next() int { if !this.hasPeeked { return this.iter.next() } this.hasPeeked = false return this.peekedElement } func (this *PeekingIterator) peek() int { if !this.hasPeeked { this.peekedElement = this.iter.next() this.hasPeeked = true } return this.peekedElement }