feat: add cpp solution to lc problem: No.1721 (#786)

No.1721.Swapping Nodes in a Linked List

Author: Dhoni77

solution/1700-1799/1721.Swapping Nodes in a Linked List/README.md

@@ -123,10 +123,35 @@ class Solution {
 }
 ```
 
-### **...**
-
-```
+### **C++**
 
+```cpp
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode() : val(0), next(nullptr) {}
+ *     ListNode(int x) : val(x), next(nullptr) {}
+ *     ListNode(int x, ListNode *next) : val(x), next(next) {}
+ * };
+ */
+class Solution {
+public:
+    ListNode* swapNodes(ListNode* head, int k) {
+        ListNode *p1 = head;
+        for (int i = 1; i < k; i++)
+            p1 = p1->next;
+        ListNode* slow = head, *fast = p1->next;
+        
+        while (fast) {
+            fast = fast->next;
+            slow = slow->next;
+        }
+        swap(slow->val, p1->val);
+        return head;
+    }
+};
 ```
 
 <!-- tabs:end -->

solution/1700-1799/1721.Swapping Nodes in a Linked List/README_EN.md

@@ -92,10 +92,35 @@ class Solution {
 }
 ```
 
-### **...**
-
-```
+### **C++**
 
+```cpp
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode() : val(0), next(nullptr) {}
+ *     ListNode(int x) : val(x), next(nullptr) {}
+ *     ListNode(int x, ListNode *next) : val(x), next(next) {}
+ * };
+ */
+class Solution {
+public:
+    ListNode* swapNodes(ListNode* head, int k) {
+        ListNode *p1 = head;
+        for (int i = 1; i < k; i++)
+            p1 = p1->next;
+        ListNode* slow = head, *fast = p1->next;
+        
+        while (fast) {
+            fast = fast->next;
+            slow = slow->next;
+        }
+        swap(slow->val, p1->val);
+        return head;
+    }
+};
 ```
 
 <!-- tabs:end -->

solution/1700-1799/1721.Swapping Nodes in a Linked List/Solution.cpp

@@ -0,0 +1,26 @@
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode() : val(0), next(nullptr) {}
+ *     ListNode(int x) : val(x), next(nullptr) {}
+ *     ListNode(int x, ListNode *next) : val(x), next(next) {}
+ * };
+ */
+class Solution {
+public:
+    ListNode* swapNodes(ListNode* head, int k) {
+        ListNode *p1 = head;
+        for (int i = 1; i < k; i++)
+            p1 = p1->next;
+        ListNode* slow = head, *fast = p1->next;
+        
+        while (fast) {
+            fast = fast->next;
+            slow = slow->next;
+        }
+        swap(slow->val, p1->val);
+        return head;
+    }
+};