# LeetCode 82. Remove Duplicates from Sorted List II
contributed by < [Eric Lin](https://github.com/ericlinsechs) >

## Aim
* Provide solutions for the [LeetCode 82. Remove Duplicates from Sorted List II](https://leetcode.com/problems/remove-duplicates-from-sorted-list-ii/)

## Recursive method

* `line 13`: If the next node exists and its value is not a duplicate, proceed to `line 20`.
* `line 15`: When a duplicate value is found, skip the current node and advance to the next node in the list.
* `line 17`: After skip all consecutive nodes with the same value, continue traversing the remaining nodes of the linked list.
* `line 20`: Establish links for the subsequent nodes in the processed list.

:::warning
Notice that `A && (B == C)` is equal to `A && B == C` because `==` has a higher precedence than `&&` according to [C Operator Precedence](https://en.cppreference.com/w/c/language/operator_precedence).
:::

#include <stddef.h>

struct ListNode {
    int val;
    struct ListNode *next;
};

struct ListNode *deleteDuplicates(struct ListNode *head)
{
    if (!head)
        return NULL;

    if (head->next && head->val == head->next->val) {
        /* Remove all duplicate numbers */
        while (head->next && head->val == head->next->val)
            head = head->next;
        return deleteDuplicates(head->next);
    }

    head->next = deleteDuplicates(head->next);
    return head;
}

> source: [2022q1 week 1 quiz](https://hackmd.io/@sysprog/linux2022-quiz1#%E6%B8%AC%E9%A9%97-2)

## Iterative method
* `line 13`: Initial a double pointer `**curr`to the address of the `head`. This allows manipulation of the original list while traversing it.
* `line 18`: If duplicates are found, skip all consecutive nodes with the same value.
* `line 20`: Adjust the `*curr` to point to the next node with distinct value.
* `line 22`: If no duplicates are found, move the `**curr` to the next node, continuing the traversal.

:::warning
Using a double pointer allows you to modify the pointer that points to the current node without affecting the original head pointer.
:::

#include <stddef.h>

struct ListNode {
    int val;
    struct ListNode *next;
};

struct ListNode *deleteDuplicates(struct ListNode *head)
{
    if (!head)
        return NULL;

    struct ListNode **curr = &head;

    while (*curr && (*curr)->next) {
        if ((*curr)->val == (*curr)->next->val) {
            /* Remove all duplicate numbers */
            while ((*curr)->next && (*curr)->val == (*curr)->next->val)
                *curr = (*curr)->next;
            *curr = (*curr)->next;
        } else {
            curr = &(*curr)->next;
        }
    }
    return head;
}

:::info
TODO: Circular doubly-link list
:::