203. 移除链表元素 力扣

/**
 * 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* removeElements(ListNode* head, int val) {
        
        if (head==nullptr) return head;  // easy mistake ignore输入就为空

        while( head!=nullptr && head->val==val)  head=head->next; //一直next有可能会到空
        ListNode* headnew=head;
        
        ListNode* pre=head;
        while(head!=nullptr)
        {
            if (head->val!=val)
            {
                pre=head;
                head=head->next;
            } 
            else
            {
               pre->next=head->next;
               head=head->next;
            }
        }
        return headnew;
    }
};