Remove Duplicates from Sorted List

Given a sorted linked list, delete all duplicates such that each element appear onlyonce.

Example 1:

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

Example 2:

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

Solution

Typical Two Pointers Approach ()

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        ListNode p, q;
        p = head;
        q = head;
        while (q != null) {
            if (q.val != p.val) {
                p.next = q;
                p = p.next;
            }
            q = q.next;
        }

        if (p != null) {
            p.next = null;
        }
        return head;
    }
}

Even Simpler Approach - Time O(n), Space O(1) - LeetCode official (0ms, 100%)

All nodes in the list up to the pointer current do not contain duplicate elements.

public ListNode deleteDuplicates(ListNode head) {
    ListNode current = head;
    while (current != null && current.next != null) {
        if (current.next.val == current.val) {
            current.next = current.next.next;
        } else {
            current = current.next;
        }
    }
    return head;
}