83. Remove Duplicates from Sorted List

Question

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.

---

Approach

1. If the head is empty, return.
2. Traverse the nodes, and compae its values to the next node.
3. If the values are the same, save the next->next node for later.
4. Delete the next node (since it is a duplicate).
5. Link the node saved just now to the next of current node.
6. Only move one node forward if there is no deletion of nodes.

Solution

/**
 * 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) {
        if(head == NULL) return head;
        
        ListNode* curr = head;
        ListNode* next = NULL;

        while(curr -> next != NULL){
            if(curr->val == curr->next->val){
                next = curr->next->next;
                delete curr->next;
                curr->next = next;
            }else{
                curr = curr->next;
            }
        }
        return head;
    }
};