Create 0148-sort-list.cpp

cpp/0148-sort-list.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 {
+private:
+ // find middle of LL using slow and fast pointers
+ ListNode* findLLMid(ListNode* head) {
+ ListNode* slow = head;
+ ListNode* fast = head->next;
+ while (fast != NULL && fast->next != NULL) {
+ slow = slow->next;
+ fast = fast->next->next;
+ }
+ return slow;
+ }
+
+ // merges two sorted LLs
+ ListNode* merge(ListNode* head1, ListNode* head2) {
+ ListNode* dummy = new ListNode();
+ ListNode* p = dummy;
+ ListNode* p1 = head1;
+ ListNode* p2 = head2;
+
+ while (p1 != NULL && p2 != NULL) {
+ if (p1->val < p2->val) {
+ ListNode* temp = p1->next;
+ p->next = p1;
+ p1->next = NULL;
+ p1 = temp;
+ } else {
+ ListNode* temp = p2->next;
+ p->next = p2;
+ p2->next = NULL;
+ p2 = temp;
+ }
+ p = p->next;
+ }
+
+ if (p1 == NULL) {
+ p->next = p2;
+ } else if (p2 == NULL) {
+ p->next = p1;
+ }
+
+ return dummy->next;
+ }
+public:
+ // merge sort implementation
+ ListNode* sortList(ListNode* head) {
+ // base cases
+ if (head == NULL)
+ return NULL;
+ if (head->next == NULL) 
+ return head;
+
+ // split LL into two halves
+ ListNode* middleOfLL = findLLMid(head);
+ ListNode* leftHalf = head;
+ ListNode* rightHalf = middleOfLL->next;
+ middleOfLL->next = NULL; // this cuts off the right half from the left half
+
+ // sort the two halves seperately and then merge them into one 
+ leftHalf = sortList(leftHalf); 
+ rightHalf = sortList(rightHalf);
+ head = merge(leftHalf, rightHalf);
+
+ return head;
+ }
+};