284. Peeking Iterator

Description

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 iterator iterator.
• int next() Returns the next element in the array and moves the pointer to the next element.
• boolean hasNext() Returns true 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 and peek are valid.
• At most 1000 calls will be made to next, hasNext, and peek.

 

Follow up: How would you extend your design to be generic and work with all types, not just integer?

Solutions

Solution 1

Python3JavaC++Go

# 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].

// 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();
    }
}

/*
 * 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;
};

/*   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
}

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