leetcode 83. pay attention to the second while loop which could be result in null ListNode struct so NULL check is required.

83.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) {
        if (head == NULL || head->next == NULL) {
            return head;
        }
        ListNode* node = head;
        while(node != NULL && node->next != NULL) {
            while (node->next != NULL && node->next->val == node->val) {
                ListNode* n = node->next;
                node->next = node->next->next;
                delete n;
            }
            node = node->next;
        }
        return head;
    }
};