1.反转链表

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func reverseList(head *ListNode) *ListNode {
    var pre *ListNode
    for head !=nil{
        next:=head.Next
        head.Next=pre
        pre=head
        head=next
    }
    return pre
}