链表——移除链表元素

题目顺序：代码随想录算法公开课，b站上有相应视频讲解

一、题目

203. Remove Linked List Elements

Given the head of a linked list and an integer val, remove all the nodes of the linked list that has Node.val == val, and return the new head.

Example 1:

Input: head = [1,2,6,3,4,5,6], val = 6
Output: [1,2,3,4,5]
Example 2:

Input: head = [], val = 1
Output: []
Example 3:

Input: head = [7,7,7,7], val = 7
Output: []

Constraints:

The number of nodes in the list is in the range [0, 104].
1 <= Node.val <= 50
0 <= val <= 50

二、题解

/**
 * 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) {
        //如果删除的节点是头结点
        while(head != NULL && head->val == val){
            ListNode * tmp = head;
            head = head -> next;
            delete tmp;
        }
        //如果是中间节点
        ListNode * cur = head;
        while(cur != NULL && cur->next != NULL){
            if(cur->next->val == val) {
                ListNode * tmp = cur->next;
                cur->next = cur->next->next;
                delete tmp;
            }
            else cur = cur -> next;
        }
        return head;
    }
};