add: 024

Author: Blankj

README.md

@@ -80,6 +80,7 @@
 | 18   | [4Sum][018]                              | Array, Hash Table, Two Pointers  |
 | 19   | [Remove Nth Node From End of List][019]  | Linked List, Two Pointers        |
 | 22   | [Generate Parentheses][022]              | String, Backtracking             |
+| 24   | [Swap Nodes in Pairs][024]               | Linked List                      |
 | 33   | [Search in Rotated Sorted Array][033]    | Arrays, Binary Search            |
 | 43   | [Multiply Strings][043]                  | Math, String                     |
 | 49   | [Group Anagrams][049]                    | Hash Table, String               |
@@ -154,6 +155,7 @@
 [018]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/018/README.md
 [019]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/019/README.md
 [022]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/022/README.md
+[024]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/024/README.md
 [033]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/033/README.md
 [043]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/043/README.md
 [049]: https://github.com/Blankj/awesome-java-leetcode/blob/master/note/049/README.md

note/024/README.md

@@ -0,0 +1,77 @@
+# [Swap Nodes in Pairs][title]
+
+## Description
+
+Given a linked list, swap every two adjacent nodes and return its head.
+
+For example,
+Given `1->2->3->4`, you should return the list as `2->1->4->3`.
+
+Your algorithm should use only constant space. You may **not** modify the values in the list, only nodes itself can be changed.
+
+**Tags:** Linked List
+
+
+## 思路 0
+
+题意是让你交换链表中相邻的两个节点，最终返回交换后链表的头，限定你空间复杂度为 O(1)。我们可以用递归来算出子集合的结果，递归的终点就是指针指到链表末少于两个元素时，如果不是终点，那么我们就对其两节点进行交换，这里我们需要一个临时节点来作为交换桥梁，就不多说了。
+
+```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 node = head.next;
+        head.next = swapPairs(node.next);
+        node.next = head;
+        return node;
+    }
+}
+```
+
+
+## 思路 1
+
+另一种实现方式就是用循环来实现了，两两交换节点，也需要一个临时节点来作为交换桥梁，直到当前指针指到链表末少于两个元素时停止，代码很简单，如下所示。
+
+```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) {
+        ListNode preHead = new ListNode(0), cur = preHead;
+        preHead.next = head;
+        while (cur.next != null && cur.next.next != null) {
+            ListNode temp = cur.next.next;
+            cur.next.next = temp.next;
+            temp.next = cur.next;
+            cur.next = temp;
+            cur = cur.next.next;
+        }
+        return preHead.next;
+    }
+}
+```
+
+
+## 结语
+
+如果你同我一样热爱数据结构、算法、LeetCode，可以关注我 GitHub 上的 LeetCode 题解：[awesome-java-leetcode][ajl]
+
+
+
+[title]: https://leetcode.com/problems/swap-nodes-in-pairs
+[ajl]: https://github.com/Blankj/awesome-java-leetcode

src/com/blankj/medium/_024/Solution.java

@@ -0,0 +1,39 @@
+package com.blankj.medium._024;
+
+import com.blankj.structure.ListNode;
+
+/**
+ * <pre>
+ *     author: Blankj
+ *     blog  : http://blankj.com
+ *     time  : 2018/01/31
+ *     desc  :
+ * </pre>
+ */
+public class Solution {
+//    public ListNode swapPairs(ListNode head) {
+//        if (head == null || head.next == null) return head;
+//        ListNode node = head.next;
+//        head.next = swapPairs(node.next);
+//        node.next = head;
+//        return node;
+//    }
+
+    public ListNode swapPairs(ListNode head) {
+        ListNode preHead = new ListNode(0), cur = preHead;
+        preHead.next = head;
+        while (cur.next != null && cur.next.next != null) {
+            ListNode temp = cur.next.next;
+            cur.next.next = temp.next;
+            temp.next = cur.next;
+            cur.next = temp;
+            cur = cur.next.next;
+        }
+        return preHead.next;
+    }
+
+    public static void main(String[] args) {
+        Solution solution = new Solution();
+        ListNode.print(solution.swapPairs(ListNode.createTestData("[1,2,3,4]")));
+    }
+}