Create 148. Sort List

Author: Manikant69

Diff for: 148. Sort List

@@ -0,0 +1,89 @@
+/**
+ * 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* getMid(ListNode* head)
+    {
+        ListNode* slow = head;
+        ListNode* fast = head -> next;
+
+        while(fast != NULL && fast -> next != NULL)
+        {
+            fast = fast -> next -> next;
+            slow = slow -> next;
+        }
+
+        return slow;
+    }
+
+    ListNode* merge(ListNode* left, ListNode* right)
+    {
+        if(left == NULL)
+            return right;
+        if(right == NULL)
+            return left;
+        ListNode* ans = new ListNode(-1);
+        ListNode* temp = ans;
+
+        while(left != NULL && right != NULL)
+        {
+            if(right -> val < left -> val)
+            {
+                temp -> next = right;
+                temp = right;
+                right = right ->next;
+            }
+            else
+            {
+                temp -> next = left;
+                temp = left;
+                left = left -> next;
+            }
+        }
+
+        while(left != NULL)
+        {
+            temp -> next = left;
+            temp = left;
+            left = left -> next;
+        }
+        while(right != NULL)
+        {
+            temp -> next = right;
+            temp = right;
+            right = right ->next;
+        }
+
+        return ans -> next;
+    }
+
+    ListNode* sortList(ListNode* head) {
+        // base case
+        if(head == NULL || head -> next == NULL)
+            return head;
+
+       //divide list into two halves
+       ListNode* mid = getMid(head);
+
+       ListNode* left = head;
+       ListNode* right = mid -> next;
+       mid -> next = NULL;
+
+        //recursive calls for sort the both halves
+       left = sortList(left);
+       right = sortList(right);
+
+       //merge the halves
+       ListNode* ans = merge(left, right);
+
+       return ans;
+    }
+};