Approach: Skip equal neighbours

def deleteDuplicates(self, head):
    cur = head
    while cur and cur.next:
        if cur.next.val == cur.val:
            cur.next = cur.next.next
        else:
            cur = cur.next
    return head

Complexity: O(n) time, O(1) space.

Full Solution — all languages

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next
class Solution:
    def deleteDuplicates(self, head):
        cur = head
        while cur and cur.next:
            if cur.next.val == cur.val:
                cur.next = cur.next.next
            else:
                cur = cur.next
        return head

import java.util.*;
class ListNode {
    int val;
    ListNode next;
    ListNode(int x) {
        val = x;
    }
}
class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        ListNode cur = head;
        while (cur != null && cur.next != null) {
            if (cur.next.val == cur.val) cur.next = cur.next.next;
            else cur = cur.next;
        }
        return head;
    }
}

#include <bits/stdc++.h> using namespace std;
struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x): val(x), next(nullptr) {
    }
};
class Solution {
    public: ListNode* deleteDuplicates(ListNode* head) {
        ListNode* cur = head;
        while (cur && cur->next) {
            if (cur->next->val == cur->val) cur->next = cur->next->next;
            else cur = cur->next;
        }
        return head;
    }
};

/** Node shape: { val, next }. Build/return plain objects or use this ctor. */ function ListNode(val, next) {
    this.val = (val === undefined ? 0 : val);
    this.next = (next === undefined ? null : next);
}
/**
 * @param {ListNode} head
 * @return {ListNode}
 */ var deleteDuplicates = function(head) {
    let cur = head;
    while (cur && cur.next) {
        if (cur.next.val === cur.val) cur.next = cur.next.next;
        else cur = cur.next;
    }
    return head;
};

Remove Duplicates from Sorted List

Example 1

Example 2

Constraints