Remove Duplicates from Sorted List

Description

Given the head of a sorted linked list, delete all duplicates such that each element appears only once. Return the linked list sorted as well.

Example 1:

Input: head = [1,1,2] Output: [1,2]

Example 2:

Input: head = [1,1,2,3,3] Output: [1,2,3]

Constraints:

• The number of nodes in the list is in the range [0, 300].
• -100 <= Node.val <= 100
• The list is guaranteed to be sorted in ascending order.

Code

Indirect Pointer

Time Complexity: $O(N)$, Space Complexity: $O(1)$

不過這個 solution 並沒有 free 被刪掉的 node，會造成 memory leak。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* deleteDuplicates(struct ListNode* head) {
    struct ListNode** indirect = &head;
    while(*indirect) {
        if ((*indirect)->next && (*indirect)->val == (*indirect)->next->val) 
            *indirect = (*indirect)->next;
        else 
            indirect = &((*indirect)->next);
    }
    return head;
}

Pointer

Time Complexity: $O(N)$, Space Complexity: $O(1)$

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* deleteDuplicates(ListNode* head) {
        ListNode* cur = head;
        while(cur) {
            while(cur->next && cur->val == cur->next->val) {
                cur->next = cur->next->next;
            }
            cur = cur->next;
        }
 
        return head;
    }
};

Source

• Remove Duplicates from Sorted List - LeetCode