链表反转

class Solution {
    func reverseList(_ head:ListNode?) -> ListNode? {
        var prevNode :ListNode? = nil
        var reverseNode :ListNode？ = nil
        var node = head
        
        while (node != nil) {
            let nextNode = node?.next
            
            if (nextNode == nil) {
                reverseNode = node
            }
            
            // 反转node结点
            node?.next = prevNode
            // 保存上一个结点，
            prevNode = node
            node = nextNode
        }
    }
    return reverseNode
}