"Palindrome Linked List"

Author: lennylxx

Diff for: README.md

@@ -24,7 +24,7 @@ You can build all the files using `make` (Use MinGW GCC and GNU Make on Windows)
 
 |     | Problem                                                      | Solution           |
 | --- | ------------------------------------------------------------ | ------------------ |
-| 234 | [Palindrome Linked List]                                     |                    |
+| 234 | [Palindrome Linked List]                                     | [C](src/234.c)     |
 | 233 | [Number of Digit One]                                        |                    |
 | 232 | [Implement Queue using Stacks]                               | [C](src/232.c)     |
 | 231 | [Power of Two]                                               | [C](src/231.c)     |

Diff for: src/234.c

@@ -0,0 +1,115 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+
+struct ListNode {
+    int val;
+    struct ListNode *next;
+};
+ 
+bool isPalindrome(struct ListNode* head) {
+    if (head == NULL) return true;
+
+    struct ListNode *p = head;
+    int len = 0;
+    while (p) {
+        len++;
+        p = p->next;
+    }
+
+    int half = len / 2;
+
+    /* get the middle node */
+    p = head;
+    while (half--) {
+        p = p->next;
+    }
+
+    /* skip middle if length is odd */
+    if (len % 2 == 1) {
+        p = p->next;
+    }
+    
+    /* reverse the right half */
+    struct ListNode *prev = NULL, *next = NULL;
+    while (p) {
+        next = p->next;
+        p->next = prev;
+        prev = p;
+        p = next;
+    }
+    
+    struct ListNode *q = prev;
+    p = head;
+    half = len / 2;
+
+    while (half--) {
+        if (p->val != q->val)
+            return false;
+        p = p->next;
+        q = q->next;
+    }
+
+    return true;
+}
+
+void printList(struct ListNode* head) {
+    if (head == NULL) return;
+
+    while (head) {
+        printf("%d->", head->val);
+        head = head->next;
+    }
+    printf("N\n");
+}
+
+int main() {
+
+    struct ListNode *l1 = (struct ListNode *)calloc(4, sizeof(struct ListNode));
+    struct ListNode *l2 = (struct ListNode *)calloc(5, sizeof(struct ListNode));
+
+    struct ListNode *p = l1;
+
+    p->val = 1;
+    p->next = l1 + 1;
+    p = p->next;
+
+    p->val = 2;
+    p->next = l1 + 2;
+    p = p->next;
+
+    p->val = 2;
+    p->next = l1 + 3;
+    p = p->next;
+
+    p->val = 1;
+    p->next = NULL;
+
+    p = l2;
+    p->val = 1;
+    p->next = l2 + 1;
+    p = p->next;
+
+    p->val = 2;
+    p->next = l2 + 2;
+    p = p->next;
+
+    p->val = 3;
+    p->next = l2 + 3;
+    p = p->next;
+
+    p->val = 2;
+    p->next = l2 + 4;
+    p = p->next;
+
+    p->val = 1;
+    p->next = NULL;
+
+    printList(l1);
+    printf("%d\n", isPalindrome(l1));
+
+    printList(l2);
+    printf("%d\n", isPalindrome(l2));
+
+    return 0;
+}