83. Remove Duplicates from Sorted List
easyCollapse consecutive duplicates in a sorted singly linked list into single occurrences. A one-pass pointer-skip drill that warms you up for the harder variant (82) where every duplicated value vanishes entirely.
By Sam K., Founder, InterviewChamp.AI · Last verified
Problem
Given the head of a sorted linked list, delete all duplicates such that each element appears only once. Return the linked list sorted as well.
Constraints
The number of nodes in the list is in the range [0, 300].-100 <= Node.val <= 100The list is guaranteed to be sorted in ascending order.
Examples
Example 1
head = [1,1,2][1,2]Example 2
head = [1,1,2,3,3][1,2,3]Example 3
head = [][]Solve it now
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Hints
Progressive — try the first before opening the next.
Hint 1
Because the list is sorted, every group of duplicates is contiguous.
Hint 2
Walk the list with a single pointer. While the next node has the same value as the current node, bypass it by setting current.next = current.next.next.
Hint 3
Only advance current when current.next has a different value — otherwise you'd skip past a real new value.
Solution approach
Reveal approach
Single-pointer in-place skip. Use one pointer current starting at head. Loop while current and current.next are both non-null: if current.val == current.next.val, rewire current.next = current.next.next (skipping the duplicate without advancing current, because the new next might also be a duplicate). Otherwise, advance current = current.next. Return head — no dummy needed because the head itself is the first occurrence and is never removed. O(n) time, O(1) extra space.
Complexity
- Time
- O(n)
- Space
- O(1)
Related patterns
- linked-list
- pointer-rewiring
- in-place-mutation
Related problems
Asked at
Companies reported asking this problem (sourced from public Glassdoor, Blind, and Levels.fyi interview posts).
- Amazon
- Microsoft
- Apple
More Linked Lists practice problems
- 2. Add Two Numbers
- 19. Remove Nth Node From End of List
- 21. Merge Two Sorted Lists
- 24. Swap Nodes in Pairs
- 25. Reverse Nodes in k-Group
- 61. Rotate List
- 82. Remove Duplicates from Sorted List II
- 86. Partition List