Approach: Dummy head + skip

def removeElements(self, head, k):
    dummy = ListNode(0, head)
    cur = dummy
    while cur.next:
        if cur.next.val == k:
            cur.next = cur.next.next
        else:
            cur = cur.next
    return dummy.next

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 removeElements(self, head, k):
        dummy = ListNode(0, head)
        cur = dummy
        while cur.next:
            if cur.next.val == k:
                cur.next = cur.next.next
            else:
                cur = cur.next
        return dummy.next

import java.util.*;
class ListNode {
    int val;
    ListNode next;
    ListNode(int x) {
        val = x;
    }
}
class Solution {
    public ListNode removeElements(ListNode head, int k) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode cur = dummy;
        while (cur.next != null) {
            if (cur.next.val == k) cur.next = cur.next.next;
            else cur = cur.next;
        }
        return dummy.next;
    }
}

#include <bits/stdc++.h> using namespace std;
struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x): val(x), next(nullptr) {
    }
};
class Solution {
    public: ListNode* removeElements(ListNode* head, int k) {
        ListNode dummy(0);
        dummy.next = head;
        ListNode* cur = &dummy;
        while (cur->next) {
            if (cur->next->val == k) cur->next = cur->next->next;
            else cur = cur->next;
        }
        return dummy.next;
    }
};

/** 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
 * @param {number} k
 * @return {ListNode}
 */ var removeElements = function(head, k) {
    const dummy = new ListNode(0, head);
    let cur = dummy;
    while (cur.next) {
        if (cur.next.val === k) cur.next = cur.next.next;
        else cur = cur.next;
    }
    return dummy.next;
};

Remove Linked List Elements

Example 1

Example 2

Constraints