[LeetCode February Challange] Day 8 - Peeking Iterator

Design an iterator that supports the peek operation on a list in addition to the hasNext and the next operations.

Implement the PeekingIterator class:

• PeekingIterator(int[] nums) Initializes the object with the given integer array nums.
• int next() Returns the next element in the array and moves the pointer to the next element.
• bool hasNext() Returns true if there are still elements in the array.
• int peek() Returns the next element in the array without moving the pointer.

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

Time complexity : O(1)
Space complexity : O(1)

/*
 * 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 {
private:
    int peeked;
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.
        peeked = -1;
    }
    
    // Returns the next element in the iteration without advancing the iterator.
    int peek() {
        if (peeked == -1 && Iterator::hasNext())
            peeked = Iterator::next();
        return peeked;
    }
    
    // hasNext() and next() should behave the same as in the Iterator interface.
    // Override them if needed.
    int next() {
        int res = peeked == -1 ? Iterator::next() : peeked;
        peeked = -1;
        return res;
    }
    
    bool hasNext() const {
        return peeked != -1 || Iterator::hasNext();
    }
};