add linked list sort

Author: akgmage

Linked List/liniked_list_sort_list.cpp

@@ -0,0 +1,75 @@
+/*
+    Given the head of a linked list, return the list after sorting it in ascending order.
+    
+    Input: head = [4,2,1,3]
+    Output: [1,2,3,4]
+
+    Input: head = [-1,5,3,4,0]
+    Output: [-1,0,3,4,5]
+
+    Input: head = []
+    Output: []
+    
+
+    Constraints:
+
+    The number of nodes in the list is in the range [0, 5 * 104].
+    -105 <= Node.val <= 105
+    
+    Follow up: Can you sort the linked list in O(n logn) time and O(1) memory (i.e. constant space)?
+*/
+
+/**
+ * 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* compute_midpoint(ListNode* head){
+        if(head == NULL || head->next == NULL){
+            return head;
+        }
+        ListNode* slow = head;
+        ListNode* fast = head->next;
+        while(fast != NULL && fast->next != NULL){
+            slow = slow->next;
+            fast = fast->next->next;
+        }
+        return slow;
+    }
+    ListNode* merge_two_lists(ListNode* A, ListNode* B){
+        if(A == NULL)
+            return B;
+        else if(B == NULL)
+            return A;
+        ListNode* C = NULL;
+        if(A->val < B->val){
+            C = A;
+            C->next = merge_two_lists(A->next, B);
+        }
+        else{
+            C = B;
+            C->next = merge_two_lists(A, B->next);
+        }
+        return C;
+    }
+    ListNode* sortList(ListNode* head) {
+        if(head == NULL || head->next == NULL)
+            return head;
+        ListNode* mid = compute_midpoint(head);
+        ListNode* A = head; 
+        ListNode* B = mid->next;
+        mid->next = NULL;
+        A = sortList(A);
+        B = sortList(B);
+        
+        ListNode* C = merge_two_lists(A, B);
+        return C;
+    }
+};