DSA

Queue Using Array

Stacks & Queues. Space O(n).

August 8, 2026

Practice Link

Implement a Queue using an Array. Queries in the Queue are of the following type: (i) 1 x (a query of this type means pushing 'x' into the queue) (ii) 2 (a query of this type means to pop an element from the queue and print the popped element. If the queue is empty then return -1)

Solution#

cpp

class ArrayQueue {
public:
    int* arrayQueue;
    int firstIdx;
    int lastIdx;
    int capacity;
    ArrayQueue() {
        capacity = 1000;
        arrayQueue = new int[capacity];
        firstIdx=0;
        lastIdx=-1;
    }
    
    void push(int x) {
        if(lastIdx == capacity-1)
            return;
        arrayQueue[++lastIdx] = x;
    }
    
    int pop() {
        if(isEmpty()){
            return -1;
        }
        int ele = arrayQueue[firstIdx++];

        if(firstIdx > lastIdx){
            firstIdx=0;
            lastIdx=-1;
        }
        return ele;
    }
    
    int peek() {
        if(isEmpty())
            return -1;
        return arrayQueue[firstIdx];
    }
    
    bool isEmpty() {
        return lastIdx==-1;
    }
};

Time Complexity:

push - O(1)

pop - O(1)

Space Complexity: O(n),

DSAImplement Queue using StacksStacks & Queues. Space O(n).DSANext Greater Element IISolution - Monotonic Stack approach. Optimal — Time O(2n), Space O(n).DSALargest Rectangle in a HistogramCovers: Brute Force, Using Stack, Optimal. Optimal — Time O(n), Space O(n).

Solution#

Related articles