83. 删除排序链表中的重复元素

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var deleteDuplicates = function (head) {
  if (head === null) {
    return head
  }
  let firstNode = head
  let value = firstNode.val
  while (firstNode) {
    while (firstNode.next && value === firstNode.next.val) {
      firstNode.next = firstNode.next.next
    }
    firstNode = firstNode.next
    value = firstNode?.val
  }
  return head
}