284. Peeking Iterator

Medium

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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?

Solution : Handles None values as well

class PeekingIterator:
    def __init__(self, iterator):
        self._iterator = iterator
        self._current = None
        self._hasNext = True
        self.next()

    def peek(self):
        return self._current

    def next(self):
        current = self._current
        if self._iterator.hasNext():
            self._current = self._iterator.next()
        else:
            self._hasNext = False
            
        return current

    def hasNext(self):
        return self._hasNext

# 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.temp = self.iterator.next() if self.iterator.hasNext() else None
        

    def peek(self):
        """
        Returns the next element in the iteration without advancing the iterator.
        :rtype: int
        """
        return self.temp
        

    def next(self):
        """
        :rtype: int
        """
        value = self.temp
        self.temp = self.iterator.next() if self.iterator.hasNext() else None
        return value
        

    def hasNext(self):
        """
        :rtype: bool
        """
        return self.temp is not None
        

# 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].