83. Remove Duplicates from Sorted List

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My Solution

The Key Idea for Solving This Coding Question

C++ Code

class Solution {
public:
    ListNode *deleteDuplicates(ListNode *head) {
        if (head == nullptr || head->next == nullptr) {
            return head;
        }
        
        ListNode *prev = head, *curr = head->next;
        
        while (curr != nullptr) {
            if (curr->val == prev->val) {
                // Find a duplicate.
                ListNode *x = curr;
                prev->next = curr->next;
                curr = curr->next;
                delete x;
            } else {
                curr = curr->next;
                prev = prev->next;
            }
        }
        
        return head;
    }
};

Time Complexity

$O(n)$

$n$ is the number of nodes in the linked list referred by head.

Space Complexity

$O(1)$

C++ Code

/**
 * 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 *dummy = new ListNode(-200, head), *prev = dummy, *curr = head;
        while (curr && curr->next) {
            if (curr->val == curr->next->val) {
                curr = curr->next;
                continue;
            }
            
            if (prev->next == curr) {
                prev = prev->next;
                curr = curr->next;
            } else {
                while (prev->next != curr) {
                    ListNode *x = prev->next;
                    prev->next = x->next;
                    delete x;
                }
            }
        }
        
        while (prev->next != curr) {
            ListNode *y = prev->next;
            prev->next = y->next;
            delete y;
        }
        
        head = dummy->next;
        delete dummy;
        return head;
    }
};

Miscellaneous