XOR linked list

XOR linked list 是为了解决双向链表两个指针占用空间的问题

XOR：异或，数学符号 “^”

XOR linked list 是将节点的previous 和 next 通过XOR算出结果保存在 npx（变量，起什么名字都无所谓）。这样就可以节省一个指针的空间，若果需要previous，previous = npx ^ next;

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

struct node{
    int data;
    struct node * npx;
};

struct node * XOR(struct node * pre, struct node * next)
{
    return (struct node *)((uintptr_t)(pre) ^ (uintptr_t)(next));
}

void insert(struct node ** head_ref, int data)
{
    struct node * new_node = (struct node *)malloc(sizeof(struct node));
    new_node->data = data;
    new_node->npx = XOR(NULL, (*head_ref));

    if(NULL != (*head_ref))
    {
        struct node * next = XOR(NULL, (*head_ref)->npx);
        (*head_ref)->npx = XOR(new_node, next);
    }

    (*head_ref) = new_node;
}
void printXORlinkedlist(struct node * head_ref)
{
    struct node * current = head_ref;
    struct node * pre = NULL;
    struct node * next;

    while(NULL != current)
    {
        printf("%d  ", current->data);

        next = XOR(current->npx, pre);

        pre = current;
        current = next;
    }
}
int main(void)
{
    struct node * head = NULL;

    insert(&head, 0);
    insert(&head, 1);
    insert(&head, 2);
    insert(&head, 3);

    printXORlinkedlist(head);
    return 0;
}

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