Add solution to day 16

05- May/16- Swap Nodes in Pairs/16- Swap Nodes in Pairs (Ahmed Gamal).cpp

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+// Author: Ahmed Gamal
+
+// for this problem, we will use a recursive approach to solve it
+// we will swap the first two nodes, then we will call the function again
+
+// to achieve this, we need to some pointers
+// 1- head (function parameter): to point to the current node
+// 2- prv (function parameter): to point to the previous node of the current node (in the new linked list)
+// 3- next: to point to the next node of the current node
+// 4- next_next: to point to the next node of the next node of the current node
+
+// we will use the prv pointer to move the connection between the previous node and the current node to the next node
+// we will use the next_next pointer to move the connection between the current node and the next node to the next next node
+// then we will move the connection between the next_next node and the next node to the current node
+
+// we will call the function again with the next_next node as the current node and the current node as the previous node
+
+class Solution {
+    void solve(ListNode* head, ListNode* prv = nullptr) {
+        if(not(head and head -> next)) {
+            return;
+        }
+
+        if(prv) {
+            prv -> next = head -> next;
+        }
+        auto next = head -> next;
+        auto next_next = next -> next;
+
+        head -> next = next_next;
+        next -> next = head;
+
+        solve(next_next, head);
+    }
+public:
+    ListNode* swapPairs(ListNode* head) {
+        if(not(head and head -> next))
+            return head;
+
+        auto ret = head -> next;
+        solve(head);
+
+        return ret;
+    }
+};