[206] 反转链表

/*
 * @lc app=leetcode.cn id=206 lang=javascript
 *
 * [206] 反转链表
 */

// @lc code=start
/**
 * 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 reverseList = function (head) {
  let pre = null,
    next = head,
    curr = head
  while (curr !== null) {
    // 开始时，pre在外面，curr和next指向第一个节点，然后next先走
    // curr.next指向pre，pre移到curr，curr 移到next，继续循环
    next = curr.next
    curr.next = pre
    pre = curr
    curr = next
  }
  return pre
}
// @lc code=end