链表---判断一个链表是否为回文链表

思路1：找到中间节点然后把后面的翻转，（需要断开链表）然后比较和头节点开始的前段，最后要是后半段的游标可以走到最后说明是回文否则不是

思路2：整体翻转比较

思路3：借助一个栈存放前半段的元素，然后和后半段的比较

    public boolean isPalindrome(ListNode head) {
            if (head == null || head.next == null) {
                return true;
            }
            ListNode fast = head.next;
            ListNode slow = head;
            while (fast != null && fast.next != null ) {
                fast = fast.next.next;
                slow = slow.next;
            }
            ListNode second = slow.next;
            slow.next = null;
     
            ListNode secondhead = null;
            ListNode p = second;
            while (p != null) {
                ListNode temp = p.next;
                p.next = secondhead;
                secondhead = p;
                p = temp;
            }
     
            ListNode p1 = head;
            ListNode p2 = secondhead;
            while (p2 != null && p2.val == p1.val) {
                p1 = p1.next;
                p2 = p2.next;
            }
            if (p2 == null) {
                return true;
            } else {
                return false;
            }
        }

    递归实现
    递归的终止条件：
    当长度为0或者1的时候表明链表处于中间位置
    class Result{
        public ListNode node;
        public boolean result;
        public Result(ListNode node,boolean res) {
            this.node = node;
            this.result=res;
        }
    }
     
    public Result ispalindromeResourse(ListNode head,int length) {
            if (head == null || length == 0) {
                return new Result(null,true);
            } else if (length == 1 ){
                return new Result(head.next,true);
            } else if (length == 2) {
                return new Result(head.next.next,head.val == head.next.val);
            }
            Result res = ispalindromeResourse(head.next,length-2);
            if (!res.result || res.node==null) {
                return res;
            } else {
                res.result = head.val == res.node.val;
                res.node = res.node.next;
                return res;
            }
        }
    }
    结果就是返回值的result
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作者：哎呦、不错哦
来源：CSDN
原文：https://blog.csdn.net/l1394049664/article/details/81350146
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