Merge pull request doocs#55 from xiapengchng/dev

Update solution 024 in cpp version with recursion

Author: yanglbme

solution/024.Swap Nodes in Pairs/Solution.cpp

@@ -0,0 +1,22 @@
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode(int x) : val(x), next(NULL) {}
+ * };
+ */
+class Solution {
+public:
+    ListNode* swapPairs(ListNode* head) {
+        if(head==NULL||head->next==NULL)//递归的最小情况，链表为空，或者长度为1，直接返回
+            return head;
+        ListNode *temp=head->next;//定义temp为head->next的node
+        ListNode *nextChain=swapPairs(head->next->next);//递归调用得到head->next->next链表交换结果
+        head->next=nextChain;//head的next指向递归后的结果
+        temp->next=head;//temp也就是之前的head->next节点指向head,那么现在的head为temp
+        return temp;
+    }
+};
+        
+