Remove Duplicates from Sorted List

solution.cpp

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *deleteDuplicates(ListNode *head) {
        auto p = head;
        
        // what's the loop invariant here?
        while (p != NULL) {
            auto q = p -> next;
            while (q != NULL && q->val == p->val) {
                q = q -> next;
            }
            
            // are we required to free those deleted nodes?
            p -> next = q;
            p = q;
        }
        
        return head;
    }
};