This page looks best with JavaScript enabled

Leetcode 284 Peeking Iterator

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.

Follow up:

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

My Solution

Success Details >
Runtime: 20 ms, faster than 99.17% of Python3 online submissions for Peeking Iterator.
Memory Usage: 14.4 MB, less than 63.77% of Python3 online submissions for Peeking Iterator.


  • 題目提供舊版 Iterator 的 interface 如註解,要做的事情就是幫他加上 peek 這個 method。
  • 以下作法是讓 peek 時,呼叫舊有的 next,然後使用一個 list 作為 cache 來暫存查詢結果,是為了方便使用 pop()(可以同時 remove element 及 return ),剛好 cache 最多只會放一個所以速度不影響,但在 next 可以 one-liner 。
  • 此題的 follow up 在 python 完全不影響。
 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
# 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.it = iterator
        self.c = []

    def peek(self):
        """
        Returns the next element in the iteration without advancing the iterator.
        :rtype: int
        """
        if not self.c:
            self.c = [self.it.next()]
        return self.c[0]

    def next(self):
        """
        :rtype: int
        """
        return self.c.pop(0) if self.c else self.it.next()

    def hasNext(self):
        """
        :rtype: bool
        """
        return bool(self.c) or self.it.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].