Peekable Iterator - Google Top Interview Questions


Problem Statement :


Implement an iterator of a list of integers nums where

peek() returns the next element, without moving the iterator

next() polls the next element in the iterator

hasnext() which returns whether the next element exists

Constraints



n ≤ 100,000 where n is the number of calls to peek, next and hasnext

Example 1

Input

methods = ["constructor", "peek", "next", "hasnext", "peek", "next", "hasnext"]

arguments = [[[1, 2]], [], [], [], [], [], []]`

Output

[None, 1, 1, True, 2, 2, False]

Explanation

First we create a PeekableIterator with values [1, 2]

We peek the next element which is 1

We poll the next element which is 1

We check if the next element exists, which it does since 2 is next in the iterator.

We peek the next element which is 2

We poll the next element which is 2

We check if the next element exists which it doesn't


Solution :



title-img 




                        Solution in C++ :

class PeekableIterator {
    public:
    PeekableIterator(vector<int>& nums) {
        arr.insert(arr.end(), nums.begin(), nums.end());
        idx = 0;
    }

    int peek() {
        if (hasnext()) {
            return arr[idx];
        }

        return -1;
    }

    int next() {
        int val = -1;
        if (hasnext()) {
            val = arr[idx++];
        }

        return val;
    }

    bool hasnext() {
        return idx < arr.size();
    }

    private:
    int idx;
    vector<int> arr;
};
                    


                        Solution in Java :

import java.util.*;

class PeekableIterator {
    int curr = 0;
    int[] arr;
    public PeekableIterator(int[] nums) {
        arr = nums;
    }

    public int peek() {
        return arr[curr];
    }

    public int next() {
        int idx = curr;
        curr++;
        return arr[idx];
    }

    public boolean hasnext() {
        return curr < arr.length;
    }
}
                    


                        Solution in Python : 
                            
class PeekableIterator:
    def __init__(self, nums):
        self.nums = deque(nums)

    def peek(self):
        return self.nums[0]

    def next(self):
        return self.nums.popleft()

    def hasnext(self):
        return True if self.nums else False


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