min value get in stack

minstack.c

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

struct tval {
    int v;
    int i;
};

struct minstack {
    int n;
    struct tval value[0];
};

void push(struct minstack *s, int v) {
    struct tval *prev = s->n ? s->value + s->n - 1 : NULL;
    struct tval *next = s->value + s->n;
    if (prev) {
        int iv = s->value[prev->i].v;
        next->i = iv > v ? s->n : prev->i;
    }
    else {
        next->i = 0;
    }

    next->v = v;
    ++s->n;
}

int pop(struct minstack *s) {
    --s->n;
    return s->value[s->n].v;
}

int top(struct minstack *s) {
    return s->value[s->n - 1].v;
}

int min(struct minstack *s) {
    int i = s->value[s->n - 1].i;
    return s->value[i].v;
}

int main(void) {
    int count = 200;
    struct minstack *stack = malloc(sizeof(struct tval) * count + sizeof(int));
    stack->n = 0;

    srand((unsigned) time(NULL));
    for(int i = 0; i < count; i++) {
        int v = rand() % 50;
        printf("push %d = %d\n", i, v);
        push(stack, v);
    }

    printf("get min:%d, i=%d\n", min(stack), stack->value[stack->n - 1].i);

    free(stack);
    return 0;
}