284. Peeking Iterator

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Problem

Given an Iterator class interface with methods: next() and hasNext(), design and implement a PeekingIterator that support the peek() operation – it essentially peek() at the element that will be returned by the next call to next().

Example:

Assume that the iterator is initialized to the beginning of the list: [1,2,3].

Call next() gets you 1, the first element in the list.
Now you call peek() and it returns 2, the next element. Calling next() after that still return 2.
You call next() the final time and it returns 3, the last element.
Calling hasNext() after that should return false.

Code

import java.util.Iterator;

// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
class PeekingIterator implements Iterator<Integer> {
    Iterator<Integer> iter;
    Integer curr;
    public PeekingIterator(Iterator<Integer> iterator) {
        // initialize any member here.
        iter = iterator;
        curr = iter.next();

    }

    // Returns the next element in the iteration without advancing the iterator.
    public Integer peek() {
        return curr;
    }

    // hasNext() and next() should behave the same as in the Iterator interface.
    // Override them if needed.
    @Override
    public Integer next() {
        int temp = curr;
        if(!iter.hasNext()){
            curr = null;
        } else{
            curr = iter.next();
        }
        
        return temp;
    }

    @Override
    public boolean hasNext() {
        return iter.hasNext() || curr != null;
    }
}

# 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.iter = iterator
        # 注意使用_next因为父类有next定义了
        self._next = iterator.next()


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


    def next(self):
        """
        :rtype: int
        """
        if self._next is None:
            raise StopIteration()

        temp = self._next

        self._next = None
        if self.iter.hasNext():
            self._next = self.iter.next()

        return temp

    def hasNext(self):
        """
        :rtype: bool
        """

        return self._next 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].