C++ Queue Exercises: Find the second lowest element of a queue

Last update on August 20 2025 11:30:12 (UTC/GMT +8 hours)

12. Second Lowest Element in a Queue

Write a C++ program to find the second lowest element of a queue.

Sample Solution:

C Code:

#include <iostream>

#include <map>
#include <cmath>

using namespace std;

const int MAX_SIZE = 100;

// Class representing a Queue data structure
class Queue {
private:
    int front;
    int rear;
    int arr[MAX_SIZE];

public:
    // Constructor initializing the Queue
    Queue() {
        front = -1;
        rear = -1;
    }

    // Check if the Queue is full
    bool isFull() {
        return (rear == MAX_SIZE - 1);
    }

    // Check if the Queue is empty
    bool isEmpty() {
        return (front == -1 && rear == -1);
    }

    // Add an element to the Queue (enqueue)
    void enqueue(int x) {
        if (isFull()) {
            cout << "Error: Queue is full" << endl;
            return;
        }
        if (isEmpty()) {
            front = 0;
            rear = 0;
        } else {
            rear++;
        }
        arr[rear] = x;
    }

    // Remove an element from the Queue (dequeue)
    void dequeue() {
        if (isEmpty()) {
            cout << "Error: Queue is empty" << endl;
            return;
        }
        if (front == rear) {
            front = -1;
            rear = -1;
        } else {
            front++;
        }
    }

    // Get the front element of the Queue
    int peek() {
        if (isEmpty()) {
            cout << "Error: Queue is empty" << endl;
            return -1;
        }
        return arr[front];
    }

    // Display all elements in the Queue
    void display() {
        if (isEmpty()) {
            cout << "Error: Queue is empty" << endl;
            return;
        }
        cout << "Queue elements are: ";
        for (int i = front; i <= rear; i++) {
            cout << arr[i] << " ";
        }
        cout << endl;
    }

    // Function to find the second lowest element in the Queue
    int second_lowest(Queue q) {
        int min1 = INT_MAX;
        int min2 = INT_MAX;
        while (!q.isEmpty()) {
            int val = q.peek();
            q.dequeue();
            if (val < min1) {
                min2 = min1;
                min1 = val;
            } else if (val < min2) {
                min2 = val;
            }
        }
        return min2;
    }
};

// Main function
int main() {
    cout << "Initialize a Queue." << endl;
    Queue q;

    cout << "\nInsert some elements into the queue:" << endl;
    q.enqueue(1);
    q.enqueue(2);
    q.enqueue(3);
    q.enqueue(4);
    q.enqueue(5);
    q.display();

    int sh_el = q.second_lowest(q);
    cout << "Second lowest element of the said queue: " << sh_el << endl;

    cout << "\nInsert two more elements into the queue:" << endl;
    q.enqueue(0);
    q.enqueue(7);
    q.display();

    sh_el = q.second_lowest(q);
    cout << "Second lowest element of the said queue: " << sh_el << endl;

    return 0;
}

Sample Output:

Initialize a Queue.

Insert some elements into the queue:
Queue elements are: 1 2 3 4 5
Second lowest element of the said queue: 2

Insert two more elements into the queue:
Queue elements are: 1 2 3 4 5 0 7
Second lowest element of the said queue: 1

Flowchart:

For more Practice: Solve these Related Problems:

Write a C++ program to identify the second smallest element in a queue by tracking the two lowest values.
Write a C++ program to compute the second lowest element in a queue while ensuring the first minimum is excluded.
Write a C++ program to extract the two smallest elements from a queue and output the second lowest.
Write a C++ program to find the second lowest value in a queue using an efficient one-pass algorithm.

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