README.md

284. Peeking Iterator (Medium)

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?

Companies:
Google

Related Topics:
Array, Design, Iterator

Similar Questions:

• Binary Search Tree Iterator (Medium)
• Flatten 2D Vector (Medium)
• Zigzag Iterator (Medium)

Solution 1.

// OJ: https://leetcode.com/problems/peeking-iterator/
// Author: github.com/lzl124631x
// Time: all in O(1)
// Space: O(1) ignoring the `nums` copy in Iterator
class PeekingIterator : public Iterator {
    int _next;
    bool _hasNext = false;
    void advance() {
        _hasNext = Iterator::hasNext();
        if (_hasNext) _next = Iterator::next();
    }
public:
    PeekingIterator(const vector<int>& nums) : Iterator(nums) {
        advance();
    }
    int peek() {
        return _next;
    }
    int next() {
        int val = _next;
        advance();
        return val;
    }
    bool hasNext() const {
        return _hasNext;
    }
};