C Exercises: Implement two stacks using a single array

Last update on August 04 2025 13:04:32 (UTC/GMT +8 hours)

9. Dual Stack Array Operations

Write a C program to implement two stacks using a single array.

Sample Solution:

C Code:

#include <stdio.h>
#include <stdlib.h>

#define MAX_SIZE 100

// Global stack and top variables for two stacks
int stack[MAX_SIZE];
int top1 = -1, top2 = MAX_SIZE;

// Function to push an element onto a specific stack
void push(int stack_num, int val) {
    // Check for overflow condition
    if (top1 + 1 == top2) {
        printf("Overflow stack!\n");
        return;
    }
    // Check for stack number and push element accordingly
    if (stack_num == 1) {
        top1++;
        stack[top1] = val;
    } else {
        top2--;
        stack[top2] = val;
    }
}

// Function to pop an element from a specific stack
int pop(int stack_num) {
    // Check for stack number and pop element accordingly
    if (stack_num == 1) {
        // Check for underflow condition in stack 1
        if (top1 == -1) {
            printf("Empty stack!\n");
            return -1;
        }
        // Pop element from stack 1
        int val = stack[top1];
        top1--;
        return val;
    } else {
        // Check for underflow condition in stack 2
        if (top2 == MAX_SIZE) {
            printf("Stack Underflow\n");
            return -1;
        }
        // Pop element from stack 2
        int val = stack[top2];
        top2++;
        return val;
    }
}

int main() {
    // Push elements onto different stacks
    push(1, 1);
    push(1, 2);
    push(1, 3);
    push(2, 10);
    push(2, 20);
    push(2, 30);

    // Pop elements from different stacks and display
    printf("%d popped from stack 1\n", pop(1));
    printf("%d popped from stack 1\n", pop(1));
    printf("%d popped from stack 1\n", pop(1));
    printf("\n%d popped from stack 2\n", pop(2));
    printf("%d popped from stack 2\n", pop(2));
    printf("%d popped from stack 2\n", pop(2));

    return 0;
}

Output:

3 popped from stack 1
2 popped from stack 1
1 popped from stack 1

30 popped from stack 2
20 popped from stack 2
10 popped from stack 2

Flowchart:

Flowchart: Implement two stacks using a single array.

For more Practice: Solve these Related Problems:

Write a C program to simulate alternating push and pop operations between two stacks sharing a single array.
Write a C program to detect and handle overflow in one of the two stacks in a shared array structure.
Write a C program to merge two stacks implemented in one array when one stack becomes empty.
Write a C program to balance the load between two stacks in a single array by redistributing space dynamically.

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