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int vector_push_back(struct vector_t *a, int value) {
    if(a == NULL || a->size > a->capacity || a->ptr == NULL || a->size < 0 || a->capacity < 1) {
        return 1;
    }
    if(a->size == a->capacity) {
        int* newptr  = (int *) realloc(a->ptr,2*a->capacity*sizeof(int));
        if (newptr == NULL) {
            return 2;
        }
        a->ptr=newptr;
        a->capacity*=2;
    }
    *(a->ptr + a->size) = value;
    a->size++;
    return 0;

}