"Reorder List"

Author: lennylxx

README.md

@@ -155,7 +155,7 @@ The `☢` means that you need to have a LeetCode Premium Subscription.
 | 146 | [LRU Cache]                                                  | [C](src/146.c)     |
 | 145 | [Binary Tree Postorder Traversal]                            | [C](src/145.c)     |
 | 144 | [Binary Tree Preorder Traversal]                             | [C++](src/144.cpp) |
-| 143 | [Reorder List]                                               |                    |
+| 143 | [Reorder List]                                               | [C](src/143.c)     |
 | 142 | [Linked List Cycle II]                                       | [C](src/142.c)     |
 | 141 | [Linked List Cycle]                                          | [C](src/141.c)     |
 | 140 | [Word Break II]                                              |                    |

src/143.c

@@ -0,0 +1,95 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+struct ListNode {
+    int val;
+    struct ListNode *next;
+};
+
+struct ListNode *Helper(struct ListNode **forward, struct ListNode *node) {
+    if (node == NULL || node->next == NULL) return node;
+
+    struct ListNode *tail = Helper(forward, node->next);
+
+    if (*forward == tail || (*forward)->next == tail) return tail;
+
+    struct ListNode *t = (*forward)->next;
+    (*forward)->next = tail;
+    *forward = t;
+    tail->next = t;
+
+    return node;
+}
+
+void reorderList(struct ListNode* head) {
+    if (head == NULL) return;
+    struct ListNode *p = head;
+    struct ListNode *mid = Helper(&p, head);
+    mid->next = NULL;
+}
+
+struct ListNode *createNode(int new_data) {
+    struct ListNode *new_node = (struct ListNode *)malloc(sizeof(struct ListNode));
+    new_node->val = new_data;
+    new_node->next = NULL;
+    return new_node;
+}
+
+int main(){
+    /* test 1 */
+    struct ListNode *l1 = createNode(1);
+    struct ListNode *p = l1;
+    int i;
+    for (i = 2; i <= 5; i++) {
+        p->next = createNode(i);
+        p = p->next;
+    }
+    p->next = NULL;
+
+    printf("List  1: ");
+    p = l1;
+    while (p) {
+        printf("%d->", p->val);
+        p = p->next;
+    }
+    printf("NIL\n");
+
+    reorderList(l1);
+
+    printf("Reorder: ");
+    p = l1;
+    while (p) {
+        printf("%d->", p->val);
+        p = p->next;
+    }
+    printf("NIL\n");
+
+    /* test 2 */
+    struct ListNode *l2 = createNode(1);
+    p = l2;
+    for (i = 2; i <= 6; i++) {
+        p->next = createNode(i);
+        p = p->next;
+    }
+    p->next = NULL;
+
+    printf("List  2: ");
+    p = l2;
+    while (p) {
+        printf("%d->", p->val);
+        p = p->next;
+    }
+    printf("NIL\n");
+
+    reorderList(l2);
+
+    printf("Reorder: ");
+    p = l2;
+    while (p) {
+        printf("%d->", p->val);
+        p = p->next;
+    }
+    printf("NIL\n");
+
+    return 0;
+}