Date: 2026-06-06
Time: 18:47
This file is a self-contained solution to LeetCode #203 — Remove Linked List Elements. It owns three responsibilities: defining the linked list data structure (ListNode), implementing the removal algorithm (removeelements), and providing conversion helpers (tolist, from_list) plus a full test suite. In the broader leetcode-implementations repo, it follows the standard per-problem directory layout where solution.py holds both implementation and tests.
ListNodeA minimal singly-linked list node. Two fields: val: int and next: Optional[ListNode]. The default constructor (val=0, next=None) makes it usable as a dummy/sentinel without arguments.
remove_elements(head, val) -> Optional[ListNode]The core algorithm. Removes all nodes whose .val equals val and returns the (possibly new) head. Operates in-place by relinking pointers — no new nodes are allocated.
from_list(vals) -> Optional[ListNode]Constructs a linked list from a Python list. Uses the same dummy-head pattern as the main algorithm.
to_list(head) -> list[int]Linearizes a linked list back into a Python list for assertion-friendly comparison.
TestRemoveElementsNine test cases covering: standard removal, empty list, all-matching, head removal, tail removal, consecutive duplicates, single node (match and no-match), and no matches.
Dummy head (sentinel node). The central idiom. removeelements creates a ListNode(next=head) and walks from it, so deletions at the head position don't require special-casing — the loop body is uniform for every position. fromlist uses the same pattern for uniform list construction.
Two-pointer skip. curr always points to the node *before* the candidate for deletion (curr.next). When a match is found, curr.next is relinked to curr.next.next, effectively skipping the target node. When there's no match, curr advances. This is important: on a match, curr does not advance, because the new curr.next might also need removal.
Test helper check. A single method wraps the fromlist → removeelements → to_list pipeline so each test case is one line. This keeps tests declarative — input list, target value, expected output.
Imports: Only stdlib — unittest, typing.Optional, and from _future_ import annotations (for PEP 604-style forward references in the ListNode type annotation).
Imported by: Hundreds of test files across the repo import from this file (see Imported By list). This is surprising — ListNode, fromlist, and tolist appear to serve as the repo-wide linked list utilities, reused by every problem that involves linked lists. This makes this file a de facto shared dependency, even though it lives under a single problem's directory.
1. from_list converts a Python list into a ListNode chain.
2. remove_elements creates a dummy node pointing to head.
3. The while loop inspects curr.next on each iteration:
curr.next is bypassed via pointer relink. curr stays put to re-check the new curr.next.curr advances one node.4. Loop terminates when curr.next is None (end of list).
5. dummy.next is returned — this correctly handles the case where the original head was removed.
curr never points to a node that should be deleted. It always points to the last "kept" node, inspecting ahead via .next.val == target remains reachable from dummy.next. The non-advance on match ensures consecutive matching nodes are all removed.None. The function trusts its inputs (valid linked list, integer val). None head is handled implicitly — the while loop body never executes, and dummy.next (which is None) is returned. This is appropriate for a LeetCode solution operating within a controlled input contract.
remove-linked-list-elements/test_solution.py — The separate test file; compare with the inline tests to see if there's divergence or additional coveragereverse-linked-list/solution.py:reverse_list — Another core linked list operation using similar pointer manipulation; good for comparing iteration patternsmerge-two-sorted-lists/solution.py:mergetwolists — Uses the same dummy-head sentinel pattern in a different context (merging vs. filtering)palindrome-linked-list/solution.py — More complex linked list problem that likely reuses ListNode/fromlist/tolist from this filedummy-head-pattern — The sentinel node idiom appears across many linked list solutions in this repo; understanding it once unlocks all of themremove-elements-dummy-head — remove_elements uses a dummy sentinel node so that head-position deletions require no special caseremove-elements-no-advance-on-match — When a node is removed, curr does not advance, ensuring consecutive matching nodes are all deleted in sequencelistnode-shared-dependency — ListNode, fromlist, and tolist from this file are imported by hundreds of other problem test files as the repo's de facto linked list utilitiesremove-elements-single-pass — The algorithm completes in O(n) time with O(1) extra space, making exactly one pass through the list