203. Remove Linked List Elements

• python3

Problem

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, 104].
• 1 <= Node.val <= 50
• 0 <= val <= 50

Pseudocode

0. Make dummy head.
1. Travel the linked list.
2. If the value of the node equal to the given value, remove it.
3. Return the head.

Code

# 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 = ListNode(0)
        dummy.next = head
        current = head
        pre = dummy
        
        while current:
            next_node = current.next
            if current.val == val:
                pre.next = next_node  # Remove the current node
            else:
                pre = current  # Update pre only if current is not removed
            current = next_node
        
        return dummy.next  # Return the new head of the list

Result

• Time Complexity : O(n)
• Space Complexity : O(1)