adding merge sort of list exercise and code

leetcode/p148_sortlist.cpp

@@ -0,0 +1,76 @@
+/**
+ * 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* mergeLists(ListNode* head1, ListNode* head2){
+        // the value doesn't matter here
+        ListNode *head = new ListNode(-1);
+        ListNode* current;
+        current = head;
+        if(head1 == nullptr){
+            return head2;
+        }
+        if(head2 == nullptr){
+            return head1;
+        }
+        while(head1!=nullptr && head2!=nullptr){
+            if(head1->val < head2->val){
+                current->next = head1;
+                head1 = head1->next;
+            }else{
+                current->next = head2;
+                head2 = head2->next;
+            }
+            current = current->next;
+        }
+
+        // at this point, one of them has already been merged into the "New" list.
+        if(head1!=nullptr){
+            // this means we still have some elements on the 1st list, now let's merge them into the "New" list.
+                current->next = head1;
+        }else{
+            // this means we still have some elements on the 2nd list, now let's merge them into the "New" list.
+                current->next = head2;
+        }
+        return head->next;
+    }
+
+    ListNode* sortList(ListNode* head) {
+        // base case
+        if(head==nullptr || head->next==nullptr){
+            return head;
+        }
+
+        // split the list
+        ListNode* slow=head;
+        ListNode* fast=head;
+        ListNode* tail1;
+        while(fast!=nullptr && fast->next!=nullptr){
+            tail1 = slow;
+            slow = slow->next;
+            fast = fast->next;
+            fast = fast->next;
+        }
+        // this is the actual split action
+        tail1->next = nullptr;
+        // slow actually is now pointing to the head of the second half
+        // so let's just rename it to make the variable name more meaningful
+        ListNode* head2 = slow;
+
+        // now let's do the recursion
+        head = sortList(head);
+        head2 = sortList(head2);
+
+        // after the two sub-lists are sorted, we merge them into one list
+        head = mergeLists(head, head2);
+        return head;
+    }
+};