203. Remove Linked List Elements

方法1: iterative

class Solution {
public:
    ListNode* removeElements(ListNode* head, int val) {
        if (!head) return head;
        ListNode * prev = new ListNode(-1);
        ListNode * newHead = prev;
        prev -> next = head;
        while (head){
            if (head -> val == val) {
                prev -> next = head -> next;
                head = head -> next;
            }
            else {
                 prev = prev -> next;
                head = head -> next;
            }
           
        }
        return newHead->next;
    }
};

方法2: recursion

class Solution {
public:
    ListNode* removeElements(ListNode* head, int val) {
        if (!head) return head;
        head -> next = removeElements(head -> next, val);
        if (head -> val == val){
            return head -> next;
        }
        return head;
    }
};