Problem 284 Peeking Iterator
Table of Contents
Problem Statement
Link - Problem 284
Question
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 iteratoriterator.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`.
Solution
/*
* 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 {
public:
int currNext=-1;
bool currHasNext=false;
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.
currHasNext = Iterator::hasNext();
if(currHasNext){
currNext = Iterator::next();
}
}
// Returns the next element in the iteration without advancing the iterator.
int peek() {
return currNext;
}
// hasNext() and next() should behave the same as in the Iterator interface.
// Override them if needed.
int next() {
int result = currNext;
currHasNext = Iterator::hasNext();
if(currHasNext){
currNext = Iterator::next();
}
return result;
}
bool hasNext() const {
return currHasNext;
}
};
Complexity Analysis
| Algorithm | Time Complexity | Space Complexity |
| --------- | --------------- | ---------------- |
| Peek | O(1) | O(1) |
Explanation
1. Intuition
- the given class `iterator` has the following functions:
- `int next()`
- `boolean hasNext()`
- `int next()` will return the next element and increments the iterator.
- `boolean hasNext()` will return a boolean value if there is an element present next
- We need to implement a `peek` function that will return the next element without incrementing the iterator.
- Hence we need to cache the next element and the boolean value if there is an element present next.
- We will use the `currNext` and `currHasNext` variables to store the next element and the boolean value if there is an element present next.
- We will initialize the `currNext` and `currHasNext` variables in the constructor.
- In the constructor, we will check if there is an element present next and store the next element in the `currNext` variable.
- In the `peek` function, we will return the `currNext` variable.
- In the `next` function, we will return the `currNext` variable and update the `currNext` and `currHasNext` variables.
- In the `hasNext` function, we will return the `currHasNext` variable.
2. Implementation
- We will create a class `PeekingIterator` that will inherit the `Iterator` class.
- We will create two variables `currNext` and `currHasNext` to store the next element and the boolean value if there is an element present next.
- In the constructor, we will initialize the `currNext` and `currHasNext` variables.
- In the `peek` function, we will return the `currNext` variable.
- In the `next` function
- We will store the `currNext` variable in the `result` variable.
- if there is an element present next, we will update the `currNext` and `currHasNext` variables.
- return `result`
- In the `hasNext` function return the `currHasNext` variable.