反转链表

双指针反转

private static ListNode revertNode(ListNode head) {
    ListNode pre = null;
    ListNode cur = head;
    ListNode next;
    while（cur != null）{
        next = cur.next;
        cur.next = pre;
        pre = cur;
        cur = next;
    }
    return pre;
}

栈方式

private static ListNode revertNode(ListNode head){
    Stack<ListNode> stack = new Stack<>();
    while(head != null){
        stack.push(head);
        head = head.next;
    }
    ListNode dummyHead = new ListNode(0);
    ListNode cur = dummyHead;
    while(!stack.isEmpty()){
        cur.next = new ListNode(stack.pop().val);
        cur = cur.next;
    }
    return dummyHead.next;
}