/**
 * 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}
 */
 1、逐个断键，将后一个节点放到前面
var reverseList = function(head) {
  var len = getLen(head)
  var h = head
  var pre = head
  while(len>0){
    var temp = h.next
    h.next = null
    h.next = temp.next
    temp.next = null
    temp.next = pre
    pre = temp
    len--
  }
  return pre
};

function getLen(p){
  var length = 0
  while(p != null){
    length ++
    p=p.next
  }
  return length-1
}

 2、与1类似但代码较少
var reverseList = function(head) {
  var pre = null
  var current = head
  while(current != null){
    var temp = current.next
    current.next = pre
    pre = current
    current = temp

  }
  return pre
};