Peeking Iterator – LeetCode Solution Java , Python 3, Python 2 , C , C++, Best and Optimal Solutions , All you need.

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.

C++ Peeking Iterator LeetCode Solution

class PeekingIterator : public Iterator
{
public:
    PeekingIterator(const vector<int> &nums) : Iterator(nums)
    {
    }

    int peek()
    {
        return Iterator(*this).next();
    }

    int next()
    {
        return Iterator::next();
    }

    bool hasNext() const
    {
        return Iterator::hasNext();
    }
};

Java Peeking Iterator LeetCode Solution

class PeekingIterator implements Iterator<Integer> {
    
    Iterator<Integer> iter = null;
    Integer next = null;
    
	public PeekingIterator(Iterator<Integer> iterator) {
	    // initialize any member here.
	    iter = iterator;
        if(hasNext()) next = iter.next();
	}
	
    // Returns the next element in the iteration without advancing the iterator.
	public Integer peek() {
        return next;
	}
	
	// hasNext() and next() should behave the same as in the Iterator interface.
	// Override them if needed.
	@Override
	public Integer next() {
	    Integer currVal = next;
        if(iter.hasNext()) 
            next = iter.next();
        else 
            next = null;
        return currVal;
	}
	
	@Override
	public boolean hasNext() {
	    return next != null || iter.hasNext();
	}
}
 

Python 3 Peeking Iterator LeetCode Solution

class PeekingIterator(object):
    def __init__(self, iterator):
        self.iter = iterator
        self.temp = self.iter.next() if self.iter.hasNext() else None

    def peek(self):
        return self.temp

    def next(self):
        ret = self.temp
        self.temp = self.iter.next() if self.iter.hasNext() else None
        return ret

    def hasNext(self):
        return self.temp is not None

Array-1180
String-562
Hash Table-412
Dynamic Programming-390
Math-368
Sorting-264
Greedy-257
Depth-First Search-256
Database-215
Breadth-First Search-200
Tree-195
Binary Search-191
Matrix-176
Binary Tree-160
Two Pointers-151
Bit Manipulation-140
Stack-133
Heap (Priority Queue)-117
Design-116
Graph-108
Simulation-103
Prefix Sum-96
Backtracking-92
Counting-86
Sliding Window-73
Linked List-69
Union Find-66
Ordered Set-48
Monotonic Stack-47
Recursion-43
Trie-41
Binary Search Tree-40
Divide and Conquer-40
Enumeration-39
Bitmask-37
Queue-33
Memoization-32
Topological Sort-31
Geometry-30
Segment Tree-27
Game Theory-24
Hash Function-24
Binary Indexed Tree-21
Interactive-18
Data Stream-17
String Matching-17
Rolling Hash-17
Shortest Path-16
Number Theory-16
Combinatorics-15
Randomized-12
Monotonic Queue-9
Iterator-9
Merge Sort-9
Concurrency-9
Doubly-Linked List-8
Brainteaser-8
Probability and Statistics-7
Quickselect-7
Bucket Sort-6
Suffix Array-6
Minimum Spanning Tree-5
Counting Sort-5
Shell-4
Line Sweep-4
Reservoir Sampling-4
Eulerian Circuit-3
Radix Sort-3
Strongly Connected Componen-t2
Rejection Sampling-2
Biconnected Component-1

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