LeetCode #83: Remove Duplicates from Sorted List (C/C++).

easy

source: https://leetcode.com/problems/remove-duplicates-from-sorted-list/
C/C++ Solution to LeetCode problem 83. Remove Duplicates from Sorted List.

Problem

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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.

Examples

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Example 1:

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

Example 2:

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

Constraints

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• 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.

Solution

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We will use two pointers, one to explore each node, and one to keep tracking of the list and the element that points next.

• Initialize two pointers at the head.
• Move one pointer until the current node has a different value from the next node.
• Move once more this pointer.
• Point next of the other pointer to the pointer that moved first.
• Move the pointer.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
  ListNode* deleteDuplicates(ListNode* head) {    
    ListNode *f = head;
    ListNode *b = head;

    while (f) {
      if (f->next && f->val == f->next->val) {
        f = f->next;
        continue;
      }

      f = f->next;
      b->next = f;
      b = b->next;
    }

    return head;
  }
};