算法：链表

判断链表有环

思路：快慢指针

两链表合并

​​https://leetcode-cn.com/problems/add-two-numbers/​​ 思路：

1、链表 head 记录初始链表，tempHead 记录当前点的链表
2、输入l1 与 l2 长度可能不一致
3、进位记录 carry

public static ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        int carry = 0;
        ListNode head = new ListNode();
        ListNode tempHead = head;
        head.next = tempHead;
        while (l1 != null || l2 != null) {
            ListNode tempNode = new ListNode();
            int l1val = l1 == null ? 0 : l1.val;
            int l2val = l2 == null ? 0 : l2.val;
            int result = l1val + l2val + carry;

            carry = result / 10;
            tempNode.val = result % 10;

            tempHead.next = tempNode;
            tempHead = tempHead.next;
            
            if (l1 != null) {
                l1 = l1.next;
            }
            if (l2 != null) {
                l2 = l2.next;
            }
        }
        if (carry > 0) {
            ListNode tail = new ListNode(carry);
            tempHead.next = tail;
        }
        return head.next;
    }

查找单链表中的倒数第k个结点

快慢指针

如何实现一个lru

前后结点

如何定位链表尾部前面的第k个节点，写一下

双指针

单链表的反转

方式一：

public ListNode reverseList(ListNode head) {
       if(head==null) return head;
        if (head.next==null) return head;
        ListNode lastNode = reverseList(head.next);
        head.next.next = head;
        head.next = null;
        return lastNode;
    }

方式二：

public ListNode reverseList(ListNode head) {
        if (head == null) return head;
        ListNode tempHead = new ListNode(0);
        ListNode tempNext = tempHead.next;
        while (head != null) {
            ListNode curNext = head.next;
            head.next = tempNext;
            tempNext = head;
            head = curNext;
        }
        return tempNext;
    }

从尾到头打印单链表

思路： 使用栈

合并两个有序的单链表，合并之后的链表依然有序

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode prehead = new ListNode(-1);

        ListNode prev = prehead;
        while (l1 != null && l2 != null) {
            if (l1.val <= l2.val) {
                prev.next = l1;
                l1 = l1.next;
            } else {
                prev.next = l2;
                l2 = l2.next;
            }
            prev = prev.next;
        }

        // 合并后 l1 和 l2 最多只有一个还未被合并完，我们直接将链表末尾指向未合并完的链表即可
        prev.next = l1 == null ? l2 : l1;

        return prehead.next;
    }

求单链表中有效节点的个数