Merge pull request #1603 from javiermon/148-Sort-List

C Solution to Neetcode: 148 SortList

Author: a93a

c/0148-Sort-List.c

@@ -0,0 +1,70 @@
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     struct ListNode *next;
+ * };
+ */
+
+struct ListNode* merge(struct ListNode *list1, struct ListNode *list2) {
+    struct ListNode *head, *tail;
+
+    if (!list1 || !list2) {
+        if (!list1) {
+            return list2;
+        }
+        return list1;
+    }
+
+    if (list1->val < list2->val) {
+        head = list1;
+        list1 = list1->next;
+    } else {
+        head = list2;
+        list2 = list2->next;
+    }
+    tail = head;
+
+    while (list1 && list2) {
+        if (list1->val < list2->val) {
+            tail->next = list1;
+            list1 = list1->next;
+        } else {
+            tail->next = list2;
+            list2 = list2->next;
+        }
+        tail = tail->next;
+    }
+
+    if (list1) {
+        tail->next = list1;
+    }
+
+    if (list2) {
+        tail->next = list2;
+    }
+
+    return head;
+}
+
+// Split a linked list in two halfs:
+struct ListNode* split(struct ListNode* head) {
+    struct ListNode *slow = head, *fast = head, *prev = NULL;
+
+    while (fast && fast->next) {
+        prev = slow;
+        slow = slow->next;
+        fast = fast->next->next;
+    }
+
+    prev->next = NULL;
+    return slow;
+}
+
+struct ListNode* sortList(struct ListNode *head) {
+    if (!head || !head->next) {
+        return head;
+    }
+
+    return merge(sortList(head), sortList(split(head)));
+}