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1 files changed, 63 insertions, 0 deletions

diff --git a/top-interview-questions/easy/linked_list/05_palindrome_linked_list_2.cc b/top-interview-questions/easy/linked_list/05_palindrome_linked_list_2.cc
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+/**
+ * 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 {
+public:
+    // Return the length of the list.
+    size_t length(const ListNode* node) {
+        if (!node) return 0;
+        return 1 + length(node->next);
+    }
+
+    // Return the nth element of the list.
+    const ListNode* nth(const ListNode* node, size_t n) {
+        if (!node)  return nullptr;
+        if (n == 0) return node;
+        return nth(node->next, n-1);
+    }
+
+    // Reverse the list up to, and not including, the nth element.
+    // Return the head of the reversed list (nth-1 node).
+    ListNode* reverse(ListNode* node, size_t n) {
+        if (!node)       return nullptr;
+        if (!node->next) return node;
+        if (n == 1) {
+            node->next = nullptr;
+            return node;
+        }
+
+        ListNode* head = reverse(node->next, n-1);
+        node->next->next = node;
+        node->next = nullptr;
+        return head;
+    }
+
+    // Compare two lists for equality.
+    bool eq(const ListNode* a, const ListNode* b) {
+        if (!a && !b)         return true;
+        if (!a)               return false;
+        if (!b)               return false;
+        if (a->val != b->val) return false;
+        return eq(a->next, b->next);
+    }
+
+    bool isPalindrome(ListNode* head) {
+        if (!head)       return true;
+        if (!head->next) return true;
+
+        // If odd,  get the middle element.
+        // If even, get the first element of the second half.
+        const size_t len      = length(head);
+        const size_t middle   = len/2;
+        const ListNode* head1 = nth(head, middle + (len & 1));
+        const ListNode* head2 = reverse(head, middle);
+        return eq(head1, head2);
+    }
+};