Very readable solution: Running time O(n), Memory O(1)

#1 
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
ListNode* Solution::deleteDuplicates(ListNode* A) 
{
    if(A == NULL)
        return A;
    
    ListNode *curr, *itr; // create two pointers for current position and next postion
    curr = A;
    itr = curr->next; // initialize them
    
    while(curr->next != NULL) // while the iterator doesn't reach the end
    {
        if(curr->val == itr->val) 
        {
            itr = itr->next;  // if values are same, keep increasing the window
            if(itr == NULL) // if the last two elemets are same
            {
                curr->next = itr; // set the current to itr and break out
            }
        }
        else
        {
            curr->next = itr; // connect the unique elements dropping the repeated element
            curr = itr; // update current to the next unique number
            itr = itr->next; // increment the iterator
        }
    }
    return A; // return the head
}