Stacks

#define STACK_MAX 100

typedef struct {
    int items[STACK_MAX];
    int top;                 /* number of items; the next free slot */
} Stack;

void stack_init(Stack *s) { s->top = 0; }
int stack_empty(const Stack *s) { return s->top == 0; }

int push(Stack *s, int value) {
    if (s->top == STACK_MAX) return -1;     /* overflow: full */
    s->items[s->top++] = value;
    return 0;
}

int pop(Stack *s, int *out) {
    if (s->top == 0) return -1;             /* underflow: empty */
    *out = s->items[--s->top];
    return 0;
}

items: [ 7 | 3 | 9 |   |   | … ]
top = 3  → the next push goes to items[3]; pop returns items[2] (9)

int balanced(const char *s) {
    char st[256];
    int top = 0;
    for (; *s; s++) {
        char c = *s;
        if (c == '(' || c == '[' || c == '{') {
            if (top == (int) sizeof st) return 0;        /* too deep for this simple version */
            st[top++] = c;
        } else if (c == ')' || c == ']' || c == '}') {
            char want = (c == ')') ? '(' : (c == ']') ? '[' : '{';
            if (top == 0 || st[--top] != want) return 0;
        }
    }
    return top == 0;                                     /* nothing left open */
}
/* balanced("{[()]}") → 1, balanced("(]") → 0, balanced("((") → 0 */