🧹 Remove Duplicates from a Sorted Linked List – Python

Hi All,

Today I solved an important linked list problem: Removing duplicates from a sorted linked list.

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📌 Problem Statement

Given a sorted singly linked list, remove all duplicate nodes such that each element appears only once.

👉 Must be done:

• In-place
• Without using extra space

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🔍 Examples

Example 1:

Input: 2 -> 2 -> 4 -> 5

Output:

2 -> 4 -> 5

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Example 2:

Input: 2 -> 2 -> 2 -> 2 -> 2

Output:

2

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💡 Key Insight

👉 Since the list is sorted:

• Duplicate elements will always be adjacent

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💡 Approach

🔹 Traverse and Compare

• Start from head
• Compare current node with next node
• If same → skip next node
• Else → move forward

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🧠 Step-by-Step Logic

1. Start with current = head
2. While current and current.next exist:
   • If values are same:
     • Skip next node
   • Else:
     • Move to next node

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💻 Python Code

class ListNode:
    def __init__(self, val=0):
        self.val = val
        self.next = None


def removeDuplicates(head):
    current = head

    while current and current.next:
        if current.val == current.next.val:
            current.next = current.next.next  # skip duplicate
        else:
            current = current.next

    return head

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🔍 Dry Run

For:

2 -> 2 -> 4 -> 5

Steps:

• Compare 2 & 2 → remove duplicate
• Move to 4 → no duplicate
• Move to 5 → no duplicate

Final:

2 -> 4 -> 5

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🖥️ Sample Output

Input: 2 -> 2 -> 4 -> 5
Output: 2 -> 4 -> 5

Input: 2 -> 2 -> 2 -> 2 -> 2
Output: 2

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⚡ Complexity Analysis

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

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🧠 Why this is important?

• Uses linked list traversal
• Efficient in-place modification
• Common interview question

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✅ Conclusion

This problem helped me understand:

• Handling duplicates in sorted data
• Pointer manipulation in linked lists
• Writing clean in-place solutions

🚀 Must-know problem for coding interviews!

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