203. Remove Linked List Elements

题意：删除链表中等于给定值 val 的所有节点。

示例 1：
输入：head = [1,2,6,3,4,5,6], val = 6
输出：[1,2,3,4,5]

示例 2：
输入：head = [], val = 1
输出：[]

示例 3：
输入：head = [7,7,7,7], val = 7
输出：[]

使用双指针方法，设置一个虚拟节点来代表左指针，注意如何创建一个链表

Python写法

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def removeElements(self, head: Optional[ListNode], val: int) -> Optional[ListNode]:
        dummy = ListNode(float('-inf'))
        dummy.next = head
        prev, curr = dummy, dummy.next
        
        while curr:
            if curr.val == val:
                prev.next = curr.next
            else:
                prev = curr
            curr = curr.next
        return dummy.next
            

JavaScript写法

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @param {number} val
 * @return {ListNode}
 */
var removeElements = function(head, val) {
    const dummy = new ListNode(-1)
    dummy.next = head
    let prev = dummy
    let curr = head
    
    while (curr) {
        if (curr.val == val) {
            prev.next = curr.next
        } else {
            prev = curr
        } curr = curr.next
    }
    return dummy.next
};