Remove Duplicates from Sorted List II

LeetCode 82 | Difficulty: Medium

Medium

Problem Description

Given the head of a sorted linked list, delete all nodes that have duplicate numbers, leaving only distinct numbers from the original list. Return the linked list sorted as well.

Example 1:

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

Example 2:

Input: head = [1,1,1,2,3]
Output: [2,3]

Constraints:

- The number of nodes in the list is in the range `[0, 300]`.

- `-100 <= Node.val <= 100`

- The list is guaranteed to be **sorted** in ascending order.

Topics: Linked List, Two Pointers

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Approach

Linked List

Use pointer manipulation. Common techniques: dummy head node to simplify edge cases, fast/slow pointers for cycle detection and middle finding, prev/curr/next pattern for reversal.

When to use

In-place list manipulation, cycle detection, merging lists, finding the k-th node.

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Solutions

Solution 1: C# (Best: 166 ms)

Metric	Value
Runtime	166 ms
Memory	N/A
Date	2017-09-20

Solution

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode DeleteDuplicates(ListNode head) {
        if(head==null || head.next==null) return head;
        ListNode temp = head;
        ListNode dummy = new ListNode(Int32.MinValue);
        dummy.next = head;
        ListNode prev = dummy;
        while (temp!= null && temp.next != null)
        {
            if (temp.val == temp.next.val)
            {
                var data = temp.val;
                while (temp!= null && temp.val == data) { temp = temp.next; }
                prev.next = temp;
            }
            else
            {   prev.next = temp;
                prev = temp;
                if(temp!=null) temp = temp.next;
            }
            
        }
        return dummy.next;
    }
}

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Complexity Analysis

Approach	Time	Space
Two Pointers	$O(n)$	$O(1)$
Linked List	$O(n)$	$O(1)$

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Interview Tips

Key Points

• Discuss the brute force approach first, then optimize. Explain your thought process.
• Draw the pointer changes before coding. A dummy head node simplifies edge cases.