From 4558314734f1dee00cd6856059a19dcd9b48a30b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=E5=94=90=E6=A0=91=E6=A3=AE?= <14021051@buaa.edu.cn>
Date: Wed, 19 Dec 2018 10:01:00 +0800
Subject: [PATCH] Create 24. Swap Nodes in Pairs.md

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+# [24. Swap Nodes in Pairs](https://leetcode.com/problems/swap-nodes-in-pairs/)
+# 思路
+将一个链表两个两个地进行翻转。   
+为了方便我们可以设置一个头结点head_node。用p1指向当前需要翻转的第一个节点，p2指向当前需要翻转的第二个节点，并用pre指向p1的前一个节点。
+然后交换p1、p2两个节点即可，再将指针向右移进行下一次翻转。   
+时间复杂度O(n)，空间复杂度O(1)
+
+# C++
+``` C++
+class Solution {
+public:
+    ListNode* swapPairs(ListNode* head) {
+        if(head == NULL || head -> next == NULL) return head;
+        ListNode *head_node = new ListNode(0);
+        head_node -> next = head;
+        ListNode *pre = head_node, *p1 = head, *p2 = head -> next;
+        while(p2 != NULL){
+            // 翻转
+            p1 -> next = p2 -> next;
+            p2 -> next = p1;
+            pre -> next = p2;
+            // 指针右移，准备进行下一次翻转
+            pre = p1;
+            p1 = pre -> next;
+            if(p1 == NULL) break; // 到链尾了
+            p2 = p1 -> next;
+        }
+        return head_node -> next;
+    }
+};
+```