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This program demonstrates how to implement a stack using two queues such that the push operation runs in linear time (O(n)) and the pop operation runs in constant time (O(1)).
Approach
We use two queues (q1 and q2) to simulate stack behavior:
Push Operation (O(n)): The new element is enqueued into q2, and all elements from q1 are dequeued and enqueued into q2. Finally, the names of q1 and q2 are swapped.
Pop Operation (O(1)): The front element of q1 is dequeued.
C Code Implementation
#include <stdio.h>#include <stdlib.h>// Define a structure for the queuetypedef struct Queue {int *arr;int front, rear, size, capacity;} Queue;// Function to create a queueQueue* createQueue(int capacity) {Queue* queue = (Queue*)malloc(sizeof(Queue));queue->capacity = capacity;queue->front = 0;queue->size = 0;queue->rear = capacity - 1;queue->arr = (int*)malloc(capacity * sizeof(int));return queue;}// Check if the queue is emptyint isEmpty(Queue* queue) {return (queue->size == 0);}// Enqueue an elementvoid enqueue(Queue* queue, int item) {if (queue->size == queue->capacity) return; // Overflow conditionqueue->rear = (queue->rear + 1) % queue->capacity;queue->arr[queue->rear] = item;queue->size++;}// Dequeue an elementint dequeue(Queue* queue) {if (isEmpty(queue)) return -1; // Underflow conditionint item = queue->arr[queue->front];queue->front = (queue->front + 1) % queue->capacity;queue->size--;return item;}// Get the front element of the queueint front(Queue* queue) {if (isEmpty(queue)) return -1;return queue->arr[queue->front];}// Define a structure for the stacktypedef struct Stack {Queue *q1, *q2;} Stack;// Create a stack using two queuesStack* createStack(int capacity) {Stack* stack = (Stack*)malloc(sizeof(Stack));stack->q1 = createQueue(capacity);stack->q2 = createQueue(capacity);return stack;}// Push operation: O(n)void push(Stack* stack, int x) {enqueue(stack->q2, x); // Enqueue into q2// Move all elements from q1 to q2while (!isEmpty(stack->q1)) {enqueue(stack->q2, dequeue(stack->q1));}// Swap q1 and q2Queue* temp = stack->q1;stack->q1 = stack->q2;stack->q2 = temp;}// Pop operation: O(1)int pop(Stack* stack) {return dequeue(stack->q1); // Dequeue from q1}// Get the top element of the stackint top(Stack* stack) {return front(stack->q1); // Front of q1 is the top of the stack}// Main function to test the implementationint main() {Stack* stack = createStack(10);push(stack, 10);push(stack, 20);push(stack, 30);printf("Top element: %d\n", top(stack)); // Output: 30printf("Popped: %d\n", pop(stack)); // Output: 30printf("Top element: %d\n", top(stack)); // Output: 20return 0;}Copied!âś•Copy Implement Stack using Queues - GeeksforGeeks
Sep 18, 2025 · We are implementing a stack using two queues (q1 and q2). The idea is to make the push (x) operation simple, and adjust the order during pop () and top () so that the stack behavior (Last-In …
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