[LeetCode July Challange]Day20-Remove Linked List Elements

Remove all elements from a linked list of integers that have value val.

Example:

Input:  1->2->6->3->4->5->6, val = 6
Output: 1->2->3->4->5

---

solution

time complexity : O(n)
space complexity : O(1)

/**
 * 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) {
        ListNode dummyHead, *pre, *cur;
        dummyHead.next = head;
        cur = head;
        pre = &dummyHead;
        
        while (cur) {
            if (cur->val == val) {
                pre->next = cur->next;
                delete cur;
                cur = pre->next;
            } else {
                pre = cur;
                cur = cur->next;
            }
        }
        
        return dummyHead.next;
    }
};

從頭一個一個往尾巴檢查。
若有找到需要移除的node，則進行以下動作：

若該node是不需要移除的，直接跳下一個。