From acc3ac725c44e880cd691c23e09dff94b8eda6e9 Mon Sep 17 00:00:00 2001
From: whai <lkwhai@163.com>
Date: Fri, 29 Dec 2023 20:18:12 +0800
Subject: [PATCH] =?UTF-8?q?LeetCode24=20-=20=E9=93=BE=E8=A1=A8=E4=B8=A4?=
 =?UTF-8?q?=E4=B8=A4=E5=AF=B9=E6=8D=A2?=
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---
 .../whaifree/leetCode/middle/LeetCode24.java  | 113 ++++++++++++++++++
 1 file changed, 113 insertions(+)
 create mode 100644 src/main/java/cn/whaifree/leetCode/middle/LeetCode24.java

diff --git a/src/main/java/cn/whaifree/leetCode/middle/LeetCode24.java b/src/main/java/cn/whaifree/leetCode/middle/LeetCode24.java
new file mode 100644
index 0000000..3aa792b
--- /dev/null
+++ b/src/main/java/cn/whaifree/leetCode/middle/LeetCode24.java
@@ -0,0 +1,113 @@
+package cn.whaifree.leetCode.middle;
+
+import org.junit.Test;
+import cn.whaifree.leetCode.model.ListNode;
+
+/**
+ * 两两交换链表中的节点
+ * 中等
+ * 相关标签
+ * 相关企业
+ * 给你一个链表，两两交换其中相邻的节点，并返回交换后链表的头节点。你必须在不修改节点内部的值的情况下完成本题（即，只能进行节点交换）。
+ *
+ * 示例 1：
+ * 输入：head = [1,2,3,4]
+ * 输出：[2,1,4,3]
+ *
+ * 示例 2：
+ * 输入：head = []
+ * 输出：[]
+ *
+ * 示例 3：
+ * 输入：head = [1]
+ * 输出：[1]
+ *
+ */
+public class LeetCode24 {
+
+    /**
+     * Definition for singly-linked list.
+     * public class ListNode {
+     *     int val;
+     *     ListNode next;
+     *     ListNode() {}
+     *     ListNode(int val) { this.val = val; }
+     *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
+     * }
+     * pre index tmp
+     *  0 -> 1 -> 2 -> 3 -> 4 -> 5
+     *
+     * pre index tmp
+     *  0    1 -> 2 -> 3 -> 4 -> 5
+     *  ----------^
+     *
+     *       -----------
+     * pre index tmp   |
+     *  0    1 -> 2 -> 3 -> 4 -> 5
+     *  ----------^
+     *
+     *       -----------
+     * pre index tmp   |
+     *  0    1 <- 2 -> 3 -> 4 -> 5
+     *  ----------^
+     *
+     *  pre tmp  index
+     *   0-> 2 -> 1 -> 3 -> 4 -> 5
+     *
+     *
+     *           pre tmp  index
+     *  0-> 2 -> 1 -> 3 -> 4 -> 5
+     */
+    public ListNode swapPairs(ListNode head) {
+        if (head == null) {
+            return head;
+        }
+        ListNode pre = new ListNode(0, head);
+        ListNode tmp = head.next;
+        ListNode index = head;
+        head = pre;
+        while (tmp != null) {
+            pre.next = tmp;
+            index.next = tmp.next;
+            tmp.next = index;
+            pre = index;
+            index = pre.next;
+            tmp = index == null ? null : index.next;
+        }
+        return head.next;
+    }
+
+    public ListNode swapPairsD(ListNode head) {
+        if (head == null) {
+            return head;
+        }
+        return swap(head, head.next);
+    }
+
+    /**
+     * 递归的关键：
+     * 1. 结束条件
+     * 2. 递归条件（只考虑当前这一点点的情况，剩下的交给下次递归）
+     * @param pre
+     * @param after
+     * @return
+     */
+    public ListNode swap(ListNode pre, ListNode after) {
+        if (pre == null || after == null) {
+            return pre;
+        }
+        // 递归后面的指针
+        pre.next = swap(after.next, after.next == null ? null : after.next.next);
+        after.next = pre;
+
+        return after;
+    }
+
+
+    @Test
+    public void test() {
+        ListNode listNode = swapPairsD(ListNode.listNodeFromArray(new int[]{1,2,3,4,5}));
+        ListNode.printList(listNode);
+
+    }
+}