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    LeetCode link: 203. Remove Linked List Elements, difficulty: Easy.

    Given the head of a linked list and an integer val, remove all the nodes of the linked list that has Node.val == val, and return the new head.

    Example 1:

    Input: head = [1,2,6,3,4,5,6], val = 6

    Output: [1,2,3,4,5]

    Example 2:

    Input: head = [], val = 1

    Output: []

    Example 3:

    Input: head = [7,7,7,7], val = 7

    Output: []

    Constraints:

    • The number of nodes in the list is in the range [0, 10000].
    • 1 <= Node.val <= 50
    • 0 <= val <= 50

    Intuition
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    • Assume that the node to be deleted in the linked list is d, and the previous node of d is p, so p.next is d.

      To delete d, just set p.next = p.next.next.

    • Because p.next.next is used, the loop condition should be while (p.next != null) instead of while (p != null).

    • But there is no node before the head node, which means that the head node needs to be treated specially.

      Is there a way to make the head node no longer special? In this way, there is no need to treat the head specially.

      Click to view the answer

      The way is to introduce a dummy node, dummy.next = head.

    Complexity

    Time complexity

    O(N)

    Space complexity

    O(1)

    Solution in languages: Java Python C++ JavaScript C# Go Ruby

    Java # 

    /**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode removeElements(ListNode head, int val) {
        var dummyHead = new ListNode();
        dummyHead.next = head;
        var node = dummyHead;

        while (node.next != null) {
            if (node.next.val == val) {
                node.next = node.next.next;
            } else {
                node = node.next;
            }
        }

        return dummyHead.next;
    }
}

    Python # 

    # Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next

class Solution:
    def removeElements(self, head: Optional[ListNode], val: int) -> Optional[ListNode]:
        dummy_head = ListNode()
        dummy_head.next = head
        node = dummy_head

        while node.next:
            if node.next.val == val:
                node.next = node.next.next
            else:
                node = node.next

        return dummy_head.next

    C++ # 

    /**
 * 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* removeElements(ListNode* head, int val) {
        auto dummyHead = new ListNode();
        dummyHead->next = head;
        auto node = dummyHead;

        while (node->next != nullptr) {
            if (node->next->val == val) {
                auto next_node = node->next;
                node->next = node->next->next;
                delete next_node;
            } else {
                node = node->next;
            }
        }

        return dummyHead->next;
    }
};

    JavaScript # 

    /**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
var removeElements = function (head, val) {
  const dummyHead = new ListNode()
  dummyHead.next = head
  let node = dummyHead

  while (node.next != null) {
    if (node.next.val == val) {
      node.next = node.next.next
    } else {
      node = node.next
    }
  }

  return dummyHead.next
};

    C# # 

    /**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int val=0, ListNode next=null) {
 *         this.val = val;
 *         this.next = next;
 *     }
 * }
 */
public class Solution {
    public ListNode RemoveElements(ListNode head, int val) {
        var dummyHead = new ListNode();
        dummyHead.next = head;
        var node = dummyHead;

        while (node.next != null) {
            if (node.next.val == val) {
                node.next = node.next.next;
            } else {
                node = node.next;
            }
        }

        return dummyHead.next;
    }
}

    Go # 

    /**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func removeElements(head *ListNode, val int) *ListNode {
    dummyHead := &ListNode{}
    dummyHead.Next = head
    node := dummyHead

    for node.Next != nil {
        if node.Next.Val == val {
            node.Next = node.Next.Next
        } else {
            node = node.Next
        }
    }

    return dummyHead.Next
}

    Ruby # 

    # Definition for singly-linked list.
# class ListNode
#     attr_accessor :val, :next
#     def initialize(val = 0, _next = nil)
#         @val = val
#         @next = _next
#     end
# end

def remove_elements(head, val)
  dummy_head = ListNode.new
  dummy_head.next = head
  node = dummy_head

  until node.next.nil?
    if node.next.val == val
      node.next = node.next.next
    else
      node = node.next
    end
  end

  dummy_head.next
end
    Solution in languages: Java Python C++ JavaScript C# Go Ruby

    Other languages

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