LeetCode 83. Remove Duplicates from Sorted List

Question

I don't know why the result of subit is "member access within null pointer of type 'struct ListNode'". I tested the sample and the capital was correct, I don't know what is wrong with my cognition of the empty indicators, can someone help me?

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */


struct ListNode* deleteDuplicates(struct ListNode* head){
   
    struct ListNode* node = head;
    while(node->next != NULL && node != NULL){
        if(node->val == node->next->val){
            struct ListNode* duplicate = node->next;
            node->next = node->next->next;
            free(duplicate);
        }
        else node = node->next;
    }
    return head;
}

Jonathon S. · Accepted Answer · 2022-01-15 15:47:00Z

1

If the list is empty head will be NULL. In the while loop expression node is dereferenced using node->next which is evaluated before the right hand side, which checks node for NULL. You should check to see if node is NULL before evaluating the while loop and NULL checks for node should always be performed before NULL checks for node->next.

This would be my suggestion:

struct ListNode* deleteDuplicates(struct ListNode* head){
   
    struct ListNode* node = head;
    while(node != NULL){
        if(node->next == NULL)
        {
           node = node->next;
        }
        else if(node->val == node->next->val){
            struct ListNode* duplicate = node->next;
            node->next = node->next->next;
            free(duplicate);
        }
        else node = node->next;
    }
    return head;
}

edited Jan 15, 2022 at 15:47

answered Jan 15, 2022 at 15:40

Jonathon S.

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1 Comment

周志桓 Over a year ago

Thank you very much!I’m very grateful for your help!

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