DATA STRUCTURES PROGRAM • STACKS & EXPRESSIONS

Implement a Stack with Constant-Time Minimum

Learn how to implement a stack with constant-time minimum using a clear C program.

BeginnerStackLIFOpush/pop

PROBLEM UNDERSTANDING

Input and expected output

Sample input

No input required

Sample output

Minimum = 1
After pop, minimum = 1

COMPLETE C PROGRAM

Complete C implementation

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

#define CAPACITY 50
struct Stack { int values[CAPACITY]; int top; };
void stack_init(struct Stack *stack) { stack->top = -1; }
int stack_empty(const struct Stack *stack) { return stack->top < 0; }
void stack_push(struct Stack *stack, int value)
{
    if (stack->top + 1 >= CAPACITY) { puts("Stack overflow."); exit(EXIT_FAILURE); }
    stack->values[++stack->top] = value;
}
int stack_pop(struct Stack *stack)
{
    if (stack_empty(stack)) { puts("Stack underflow."); exit(EXIT_FAILURE); }
    return stack->values[stack->top--];
}
int stack_peek(const struct Stack *stack)
{
    if (stack_empty(stack)) { puts("Stack is empty."); exit(EXIT_FAILURE); }
    return stack->values[stack->top];
}

int main(void)
{
    int values[] = {5, 2, 8, 1, 4};
    struct Stack data, minimums;
    stack_init(&data); stack_init(&minimums);
    for (int index = 0; index < 5; index++) {
        stack_push(&data, values[index]);
        if (stack_empty(&minimums) || values[index] <= stack_peek(&minimums))
            stack_push(&minimums, values[index]);
    }
    printf("Minimum = %d\n", stack_peek(&minimums));
    int removed = stack_pop(&data);
    if (removed == stack_peek(&minimums)) stack_pop(&minimums);
    printf("After pop, minimum = %d\n", stack_peek(&minimums));
    return 0;
}

CURRENT STEP

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EXPECTED OUTPUT FOR THE SAMPLE

Minimum = 1
After pop, minimum = 1

0%Step 0 of 0

PROGRAM EXPLANATION

Algorithm and explanation

Read the required input values.
Maintain a second stack containing the minimum values active at each depth.
Display the computed result.

Maintain a second stack containing the minimum values active at each depth.

EFFICIENCY

Time and space complexity

Time complexity

O(1) per operation

Auxiliary space

O(n)

DEBUGGING CHECKLIST

Common mistakes

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Use the correct format specifier for every variable.

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Initialize variables before using their values.

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Check braces, semicolons and input order carefully.

Try it yourself

Practice: Run the program with the sample input, predict its output, and then test one boundary case of your own.