"Sort List"

Author: lennylxx

README.md

@@ -140,7 +140,7 @@ The `☢` means that you need to have a LeetCode Premium Subscription.
 | 151 | [Reverse Words in a String]                                  | [C](src/151.c)     |
 | 150 | [Evaluate Reverse Polish Notation]                           | [C](src/150.c)     |
 | 149 | [Max Points on a Line]                                       |                    |
-| 148 | [Sort List]                                                  |                    |
+| 148 | [Sort List]                                                  | [C](src/148.c)     |
 | 147 | [Insertion Sort List]                                        |                    |
 | 146 | [LRU Cache]                                                  | [C](src/146.c)     |
 | 145 | [Binary Tree Postorder Traversal]                            | [C](src/145.c)     |

src/148.c

@@ -0,0 +1,95 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+struct ListNode {
+    int val;
+    struct ListNode *next;
+};
+
+struct ListNode* merge(struct ListNode *left, struct ListNode *right) {
+    if (left == NULL) return right;
+    if (right == NULL) return left;
+
+    struct ListNode *ans = NULL;
+    struct ListNode **p = &ans;
+
+    while (left && right) {
+        if (left->val <= right->val) {
+            *p = left;
+            p = &((*p)->next);
+            left = left->next;
+        }
+        else {
+            *p = right;
+            p = &((*p)->next);
+            right = right->next;
+        }
+    }
+
+    if (left) {
+        *p = left;
+    }
+    else if (right) {
+        *p = right;
+    }
+
+    return ans;
+}
+
+struct ListNode* sortList(struct ListNode* head) {
+    if (head == NULL) return NULL;
+    if (head->next == NULL) return head;
+
+    struct ListNode *p = head;
+    int len = 0;
+    while (p) {
+        len++;
+        p = p->next;
+    }
+
+    struct ListNode *mid = head;
+    struct ListNode *prev = NULL;
+    int i = len / 2;
+    while (i--) {
+        prev = mid;
+        mid = mid->next;
+    }
+    prev->next = NULL;
+
+    struct ListNode *left = sortList(head);
+    struct ListNode *right = sortList(mid);
+
+    return merge(left, right);
+}
+
+int main() {
+    struct ListNode *head = (struct ListNode *)calloc(5, sizeof(struct ListNode));
+    struct ListNode **p = &head;
+    int i;
+    for (i = 0; i < 5; i++) {
+        (*p)->val = 5 - i;
+        (*p)->next = *p + 1;
+        p = &((*p)->next);
+    }
+    *p = NULL;
+
+    printf("List: ");
+    struct ListNode *q = head;
+    while (q != NULL) {
+        printf("%d->", q->val);
+        q = q->next;
+    }
+    printf("N\n");
+
+    struct ListNode *ret = sortList(head);
+
+    printf("Sort result: ");
+    q = ret;
+    while (q != NULL) {
+        printf("%d->", q->val);
+        q = q->next;
+    }
+    printf("N\n");
+
+    return 0;
+}