堆疊是一種線性資料結構，遵循後進先出 （LIFO） 原則，也稱為先進後出 （FILO）。這表示新增至堆疊的最後一個元素是第一個要移除的元素。它的運作方式就像一堆實體物品，例如盤子或書籍，您只能在頂部新增或刪除物品。

在 C 語言裡可以使用陣列(array)或鏈結串列(linked list) 來實作 stack

#include <stdio.h>
#include <stdlib.h> // For malloc and free
#include <limits.h> // For INT_MIN

// A structure to represent a stack
struct Stack {
    int top;
    unsigned capacity;
    int* array;
};

// Function to create a stack of given capacity.
struct Stack* createStack(unsigned capacity) {
    struct Stack* stack = (struct Stack*)malloc(sizeof(struct Stack));
    if (!stack) {
        return NULL; 
    }
    stack->capacity = capacity;
    stack->top = -1; 
    
    stack->array = (int*)malloc(stack->capacity * sizeof(int));
    if (!stack->array) {
        free(stack); 
        return NULL;
    }
    return stack;
}

int isFull(struct Stack* stack) {
    return stack->top == stack->capacity - 1;
}

int isEmpty(struct Stack* stack) {
    return stack->top == -1;
}

void push(struct Stack* stack, int item) {
    if (isFull(stack)) {
        printf("Stack Overflow\n");
        return;
    }
    stack->array[++stack->top] = item;
    printf("%d pushed to stack\n", item);
}

int pop(struct Stack* stack) {
    if (isEmpty(stack)) {
        printf("Stack Underflow\n");
        return INT_MIN;
    }
    return stack->array[stack->top--];
}

int peek(struct Stack* stack) {
    if (isEmpty(stack)) {
        printf("Stack is empty\n");
        return INT_MIN;
    }
    return stack->array[stack->top];
}

void freeStack(struct Stack* stack) {
    if (stack) {
        free(stack->array);
        free(stack); 
    }
}

int main() {
    struct Stack* stack = createStack(5);
    if (!stack) {
        printf("Failed to create stack.\n");
        return 1;
    }

    push(stack, 10);
    push(stack, 20);
    push(stack, 30);

    printf("\nTop element is %d\n", peek(stack));

    printf("%d popped from stack\n", pop(stack));
    printf("After popping, top element is %d\n\n", peek(stack));

    push(stack, 40);
    push(stack, 50);
    push(stack, 60); 
    
    push(stack, 70); // This will cause a stack overflow

    printf("\nPopping all elements:\n");
    while(!isEmpty(stack)) {
        printf("%d popped from stack\n", pop(stack));
    }

    pop(stack); // This will cause a stack underflow

    freeStack(stack);

    return 0;
}