A doubly linked list that only stores 1 pointer per element
This is an xor linked list:

struct node {
  int myValue;
  uintptr_t xor_prev_and_next;
  node *next(node *prev) {
    return (node*)((uintptr_t)prev ^ xor_prev_and_next);
  }
  node *prev(node *next) {
    return (node*)((uintptr_t)next ^ xor_prev_and_next);
  }
}

struct list {
  node *first;
  node *last;
  int sum_forwards() {
    int sum = 0;
    node *prev = NULL;
    node *curr = first;
    while (curr) {
      sum += curr-&gt;myValue;
      prev, curr = curr, curr-&gt;next(prev);
    }
    return sum;
  }
}