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https://github.com/CyC2018/Interview-Notebook.git
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Update Leetcode 题解 - 链表.md
补充 K 个一组翻转链表、重排链表、合并 K 个有序链表、链表排序，并配图说明 K 个一组翻转时的指针变化。
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CyC2018
@@ -11,6 +11,10 @@
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* [8. 回文链表](#8-回文链表)
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* [9. 分隔链表](#9-分隔链表)
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* [10. 链表元素按奇偶聚集](#10-链表元素按奇偶聚集)
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* [11. K 个一组翻转链表](#11-k-个一组翻转链表)
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* [12. 重排链表](#12-重排链表)
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* [13. 合并 K 个有序链表](#13-合并-k-个有序链表)
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* [14. 链表排序](#14-链表排序)
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<!-- GFM-TOC -->
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@@ -363,3 +367,170 @@ public ListNode oddEvenList(ListNode head) {
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return head;
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}
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```
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## 11. K 个一组翻转链表
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25\. Reverse Nodes in k-Group (Hard)
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[Leetcode](https://leetcode.com/problems/reverse-nodes-in-k-group/) / [力扣](https://leetcode.cn/problems/reverse-nodes-in-k-group/)
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```text
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Input: 1->2->3->4->5, k = 2
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Output: 2->1->4->3->5
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```
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题目描述：每 k 个节点一组进行翻转，最后不足 k 个的节点保持原有顺序。
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使用一个虚拟头节点 dummy，pre 指向下一组的前一个节点。先从 pre 向后走 k 步找到这一组的尾节点 tail，走不到说明剩余节点不足 k 个。然后把 pre.next 到 tail 这一段原地翻转，再把它接回原链表，最后 pre 移到这一组翻转后的末尾。
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<div align="center"> <img src="pics/reverse-k-group.png" width="560px"/> </div><br>
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```java
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public ListNode reverseKGroup(ListNode head, int k) {
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ListNode dummy = new ListNode(-1);
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dummy.next = head;
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ListNode pre = dummy;
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while (true) {
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ListNode tail = pre;
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for (int i = 0; i < k && tail != null; i++) {
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tail = tail.next;
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}
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if (tail == null) {
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break;
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}
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ListNode first = pre.next, next = tail.next;
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ListNode prev = next, cur = first;
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while (cur != next) {
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ListNode tmp = cur.next;
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cur.next = prev;
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prev = cur;
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cur = tmp;
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}
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pre.next = tail;
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pre = first;
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}
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return dummy.next;
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}
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```
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## 12. 重排链表
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143\. Reorder List (Medium)
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[Leetcode](https://leetcode.com/problems/reorder-list/) / [力扣](https://leetcode.cn/problems/reorder-list/)
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```text
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Input: 1->2->3->4->5
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Output: 1->5->2->4->3
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```
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题目描述：把 L0→L1→…→Ln 重排为 L0→Ln→L1→Ln-1→…，只能修改节点的指针。
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可以拆成三个已经解决过的子问题：用快慢指针找到中点并断开；把后半段反转；再把两段交替连接起来。
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```java
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public void reorderList(ListNode head) {
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if (head == null || head.next == null) {
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return;
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}
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ListNode slow = head, fast = head;
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while (fast.next != null && fast.next.next != null) {
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slow = slow.next;
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fast = fast.next.next;
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}
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ListNode l2 = reverse(slow.next);
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slow.next = null;
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ListNode l1 = head;
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while (l2 != null) {
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ListNode n1 = l1.next, n2 = l2.next;
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l1.next = l2;
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l2.next = n1;
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l1 = n1;
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l2 = n2;
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}
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}
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private ListNode reverse(ListNode head) {
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ListNode pre = null;
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while (head != null) {
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ListNode next = head.next;
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head.next = pre;
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pre = head;
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head = next;
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}
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return pre;
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}
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```
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## 13. 合并 K 个有序链表
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23\. Merge k Sorted Lists (Hard)
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[Leetcode](https://leetcode.com/problems/merge-k-sorted-lists/) / [力扣](https://leetcode.cn/problems/merge-k-sorted-lists/)
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用小顶堆保存每个链表当前的头节点，每次取出值最小的节点接到结果链表后面，再把它的下一个节点放入堆中。设一共有 N 个节点，每个节点进出堆一次，时间复杂度为 O(NlogK)。
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也可以像归并排序那样两两合并，时间复杂度同样是 O(NlogK)。
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```java
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public ListNode mergeKLists(ListNode[] lists) {
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PriorityQueue<ListNode> pq = new PriorityQueue<>((a, b) -> Integer.compare(a.val, b.val));
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for (ListNode node : lists) {
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if (node != null) {
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pq.add(node);
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}
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}
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ListNode dummy = new ListNode(-1), tail = dummy;
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while (!pq.isEmpty()) {
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ListNode node = pq.poll();
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tail.next = node;
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tail = node;
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if (node.next != null) {
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pq.add(node.next);
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}
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}
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return dummy.next;
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}
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```
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## 14. 链表排序
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148\. Sort List (Medium)
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[Leetcode](https://leetcode.com/problems/sort-list/) / [力扣](https://leetcode.cn/problems/sort-list/)
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题目描述：在 O(NlogN) 时间复杂度内对链表进行排序。
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链表不能随机访问，适合用归并排序：用快慢指针找到中点断开，两半分别排序后，用第 3 题的方法归并。递归需要 O(logN) 的栈空间。
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```java
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public ListNode sortList(ListNode head) {
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if (head == null || head.next == null) {
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return head;
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}
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ListNode slow = head, fast = head.next;
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while (fast != null && fast.next != null) {
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slow = slow.next;
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fast = fast.next.next;
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}
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ListNode right = slow.next;
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slow.next = null;
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return merge(sortList(head), sortList(right));
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}
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private ListNode merge(ListNode l1, ListNode l2) {
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ListNode dummy = new ListNode(-1), tail = dummy;
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while (l1 != null && l2 != null) {
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if (l1.val <= l2.val) {
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tail.next = l1;
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l1 = l1.next;
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} else {
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tail.next = l2;
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l2 = l2.next;
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}
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tail = tail.next;
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}
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tail.next = l1 != null ? l1 : l2;
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return dummy.next;
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}
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```
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<style>
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.title { font-weight: bold; font-size: 15px; fill: #777; text-anchor: middle; }
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.b { font-weight: bold; }
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.c { text-anchor: middle; }
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.e { text-anchor: end; }
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.note { fill: #C0605A; }
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.ok { fill: #6A9F68; }
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.dash { stroke-dasharray: 5 4; }
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.line { stroke: #666; stroke-width: 1.3; fill: none; }
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.frame { fill: none; stroke: #999; stroke-width: 1; stroke-dasharray: 4 3; }
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.shadow { fill: #C0C0C0; }
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</style>
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<defs>
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<marker id="a" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse">
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<path d="M1,1 L9,5 L1,9" fill="none" stroke="#666" stroke-width="1.6"/>
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</marker>
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<marker id="r" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse">
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<path d="M1,1 L9,5 L1,9" fill="none" stroke="#C0605A" stroke-width="1.6"/>
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</marker>
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<marker id="g" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse">
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<path d="M1,1 L9,5 L1,9" fill="none" stroke="#6A9F68" stroke-width="1.6"/>
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</marker>
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<rect width="100%" height="100%" fill="#FFFFFF"/>
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<text x="300" y="28" class="title">K 个一组翻转链表（k = 2）</text>
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<text x="20" y="90" class="b">翻转前</text>
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<rect x="88" y="72" width="50" height="32" rx="6" class="shadow"/>
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<rect x="85" y="69" width="50" height="32" rx="6" fill="#E6E6E6"/>
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<text x="110" y="89.5" class="c b">dummy</text>
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<line x1="137" y1="85" x2="163" y2="85" class="arrow "/>
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<rect x="168" y="72" width="50" height="32" rx="6" class="shadow"/>
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<rect x="165" y="69" width="50" height="32" rx="6" fill="#B1DDF0"/>
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<text x="190" y="89.5" class="c b">1</text>
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<line x1="217" y1="85" x2="243" y2="85" class="arrow "/>
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<text x="270" y="89.5" class="c b">2</text>
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<line x1="297" y1="85" x2="323" y2="85" class="arrow "/>
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<rect x="328" y="72" width="50" height="32" rx="6" class="shadow"/>
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<rect x="325" y="69" width="50" height="32" rx="6" fill="#B1DDF0"/>
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<text x="350" y="89.5" class="c b">3</text>
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<line x1="377" y1="85" x2="403" y2="85" class="arrow "/>
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<rect x="408" y="72" width="50" height="32" rx="6" class="shadow"/>
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<rect x="405" y="69" width="50" height="32" rx="6" fill="#B1DDF0"/>
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<text x="430" y="89.5" class="c b">4</text>
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<line x1="457" y1="85" x2="483" y2="85" class="arrow "/>
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<rect x="488" y="72" width="50" height="32" rx="6" class="shadow"/>
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<rect x="485" y="69" width="50" height="32" rx="6" fill="#B1DDF0"/>
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<text x="510" y="89.5" class="c b">5</text>
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<line x1="110" y1="131" x2="110" y2="105" class="arrow "/>
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<text x="110" y="147" class="c ">pre</text>
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<line x1="190" y1="131" x2="190" y2="105" class="arrow "/>
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<text x="190" y="147" class="c ">first</text>
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<line x1="270" y1="131" x2="270" y2="105" class="arrow "/>
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<text x="270" y="147" class="c ">tail</text>
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<line x1="350" y1="131" x2="350" y2="105" class="arrow "/>
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<text x="350" y="147" class="c ">next</text>
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<text x="20" y="205" class="b">翻转后</text>
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<rect x="88" y="187" width="50" height="32" rx="6" class="shadow"/>
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<rect x="85" y="184" width="50" height="32" rx="6" fill="#E6E6E6"/>
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<text x="110" y="204.5" class="c b">dummy</text>
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<line x1="137" y1="200" x2="163" y2="200" class="arrow "/>
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<rect x="168" y="187" width="50" height="32" rx="6" class="shadow"/>
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<rect x="165" y="184" width="50" height="32" rx="6" fill="#B1DDF0"/>
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<text x="190" y="204.5" class="c b">2</text>
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<line x1="217" y1="200" x2="243" y2="200" class="arrow "/>
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<rect x="248" y="187" width="50" height="32" rx="6" class="shadow"/>
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<rect x="245" y="184" width="50" height="32" rx="6" fill="#B1DDF0"/>
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<text x="270" y="204.5" class="c b">1</text>
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<line x1="297" y1="200" x2="323" y2="200" class="arrow "/>
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<rect x="328" y="187" width="50" height="32" rx="6" class="shadow"/>
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<rect x="325" y="184" width="50" height="32" rx="6" fill="#B1DDF0"/>
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<text x="350" y="204.5" class="c b">3</text>
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<line x1="377" y1="200" x2="403" y2="200" class="arrow "/>
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<rect x="408" y="187" width="50" height="32" rx="6" class="shadow"/>
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<rect x="405" y="184" width="50" height="32" rx="6" fill="#B1DDF0"/>
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<text x="430" y="204.5" class="c b">4</text>
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<line x1="457" y1="200" x2="483" y2="200" class="arrow "/>
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<rect x="488" y="187" width="50" height="32" rx="6" class="shadow"/>
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<rect x="485" y="184" width="50" height="32" rx="6" fill="#B1DDF0"/>
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||||
<text x="510" y="204.5" class="c b">5</text>
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<line x1="110" y1="246" x2="110" y2="220" class="arrow "/>
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<text x="110" y="262" class="c ">pre.next = tail</text>
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<line x1="270" y1="246" x2="270" y2="220" class="arrow green"/>
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<text x="270" y="262" class="c ok">pre = first</text>
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<text x="300" y="292" class="c sub">first 成为这一组的末尾，也是下一组的 pre</text>
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</svg>
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||||
|
After Width: | Height: | Size: 5.3 KiB |
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