leetcode -- 82

82. Remove Duplicates from Sorted List II

Problem Description

Given a sorted linked list, delete all nodes that have duplicate numbers, leaving only distinct numbers from the original list.

Return the linked list sorted as well.

Example 1:

Input: 1->2->3->3->4->4->5
Output: 1->2->5

Example 2:

Input: 1->1->1->2->3
Output: 2->3

Solution Method

在開頭重複元素卡了好久，定義一個啞節點，萬事皆休。

struct ListNode* deleteDuplicates(struct ListNode* head)
{
    if (head == NULL)
        return NULL;
    struct ListNode dummy;  // 定義一個啞節點處理開始的重複
    dummy.next = head;
    dummy.val = -1;
    struct ListNode *p = &dummy, *q = p->next, * r = q->next; 

    while (r != NULL)
    {
        if (q->val != r->val)
        {
            p->next = q;
            p = q;
            q = q->next;
        }
        else
        {
            // 跳過相同元素
            while (r->val == q->val)
            {
                r = r->next;
                if (r == NULL)
                {
                    p->next = NULL;
                    return dummy.next;
                }
            }
            // 刪除相同元素
            q = r;
            p->next = q;
        }
        r = r->next;
    }
    return dummy.next;
}