24. Swap Nodes in Pairs

Author: coderbean

src/leetcode/_22_/solution.md

@@ -47,4 +47,6 @@ class Solution {
     }
 }
         if (rigth >= 0) {
-```
+```
+
+![](https://ws1.sinaimg.cn/large/006tKfTcgy1g0vm0gawoyj30wx0u0wj1.jpg)

src/leetcode/_23_/Main.java

@@ -0,0 +1,25 @@
+package leetcode._24_;
+
+import leetcode.common.ListNode;
+
+/**
+ * Created by zhangbo54 on 2019-03-04.
+ */
+public class Main {
+    public static void main(String[] args) {
+        Solution solution = new Solution();
+        ListNode node = new ListNode(1)
+                .setNext(new ListNode(2)
+                        .setNext(new ListNode(3)
+                                .setNext(new ListNode(4))));
+        System.out.println(node);
+        System.out.println(solution.swapPairs(node));
+
+        node = new ListNode(1)
+                .setNext(new ListNode(2)
+                        .setNext(new ListNode(3)));
+        System.out.println(node);
+        System.out.println(solution.swapPairs(node));
+    }
+}
+

src/leetcode/_23_/Solution.java

@@ -0,0 +1,37 @@
+package leetcode._24_;
+
+import leetcode.common.ListNode;
+
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ * int val;
+ * ListNode next;
+ * ListNode(int x) { val = x; }
+ * }
+ */
+class Solution {
+    public ListNode swapPairs(ListNode head) {
+        if (head == null || head.next == null) {
+            return head;
+        }
+        ListNode temp = head.next;
+        head.next = temp.next;
+        temp.next = head;
+        head = temp;
+
+        ListNode frontNode = head.next.next;
+        ListNode backNode = head.next;
+        while (frontNode != null && frontNode.next != null) {
+            temp = frontNode.next;
+            backNode.next = temp;
+            frontNode.next = temp.next;
+            temp.next = frontNode;
+            backNode = backNode.next;
+
+            frontNode = frontNode.next;
+            backNode = backNode.next;
+        }
+        return head;
+    }
+}

src/leetcode/_23_/solution.md

@@ -0,0 +1,56 @@
+### [24\. Swap Nodes in Pairs](https://leetcode.com/problems/swap-nodes-in-pairs/)
+
+Difficulty: **Medium**
+
+
+Given a linked list, swap every two adjacent nodes and return its head.
+
+You may **not** modify the values in the list's nodes, only nodes itself may be changed.
+
+**Example:**
+
+```
+Given 1->2->3->4, you should return the list as 2->1->4->3.
+```
+
+
+#### Solution
+
+Language: **Java**
+
+```java
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode(int x) { val = x; }
+ * }
+ */
+class Solution {
+    public ListNode swapPairs(ListNode head) {
+        if (head == null || head.next == null) {
+            return head;
+        }
+        ListNode temp = head.next;
+        head.next = temp.next;
+        temp.next = head;
+        head = temp;
+​
+        ListNode frontNode = head.next.next;
+        ListNode backNode = head.next;
+        while (frontNode != null && frontNode.next != null) {
+            temp = frontNode.next;
+            backNode.next = temp;
+            frontNode.next = temp.next;
+            temp.next = frontNode;
+            backNode = backNode.next;
+​
+            frontNode = frontNode.next;
+            backNode = backNode.next;
+        }
+        return head;
+    }
+}
+```
+![](https://ws1.sinaimg.cn/large/006tKfTcgy1g0vlzv7nsqj310m0nstbv.jpg)