# Implementing a stack using a linked list

A “stack” is an abstract data type
that provides `push`

and `pop`

operations
which behave like “a stack of plates”.
There are many ways to implement a stack,
but a very natural way is with a “linked list”.
A linked list is a concrete data type
consisting of “nodes” in a chain.
The head of the list represents the top of the stack.
Here’s an implementation in C:

```
#include <stdlib.h>
#include <assert.h>
typedef struct Node {
int val;
struct Node * next;
} Node;
Node* new_node(int val, Node* next) {
Node* node = malloc(sizeof(Node));
node->val = val;
node->next = next;
return node;
}
// ----------------------------------------------------
// ------------------- STACK API ----------------------
typedef Node** Stack;
int pop(Stack stack) {
Node* top = *stack;
if (top == NULL) return -1;
*stack = top->next;
int x = top->val;
free(top);
return x;
}
void push(Stack stack, int x) {
*stack = new_node(x, *stack);
}
Stack new_stack() {
return calloc(1, sizeof(Node*));
}
void destroy_stack(Stack stack) {
while(pop(stack) != -1);
free(stack);
}
// ----------------------------------------------------
// --------------------- TESTS ------------------------
int main(int argc, char** argv) {
Stack stack = new_stack();
assert(pop(stack) == -1);
push(stack, 1);
assert(pop(stack) == 1);
assert(pop(stack) == -1);
push(stack, 1);
push(stack, 2);
push(stack, 3);
assert(pop(stack) == 3);
assert(pop(stack) == 2);
assert(pop(stack) == 1);
assert(pop(stack) == -1);
assert(pop(stack) == -1);
destroy_stack(stack);
}
```

Tagged #ctci, #programming, #c. All content copyright James Fisher 2020. This post is not associated with my employer.