solve 234 with C++ use O(n) space

Author: illuz

solutions/234.Palindrome_Linked_List/AC_recursive_n.cpp

@@ -0,0 +1,71 @@
+/*
+*  Author:      illuz <iilluzen[at]gmail.com>
+*  File:        AC_recursive_n.cpp
+*  Create Date: 2015-07-28 23:56:22
+*  Descripton:   
+*/
+
+#include <bits/stdc++.h>
+
+using namespace std;
+const int N = 0;
+
+// Definition for singly-linked list.
+struct ListNode {
+    int val;
+    ListNode *next;
+    ListNode(int x) : val(x), next(NULL) {}
+};
+
+class Solution {
+public:
+    bool isPalindrome(ListNode* head) {
+        int len = 0;
+        ListNode *cur = head;
+        while (cur != NULL) {
+            ++len;
+            cur = cur->next;
+        }
+        cur = head;
+        return isPalindromeHelper(cur, len);
+    }
+private:
+    bool isPalindromeHelper(ListNode* &head, int len) {
+        if (len < 2) return true;
+        if (len == 2) {
+            int head_val = head->val;
+            head = head->next;
+            int tail_val = head->val;
+            return head_val == tail_val;
+        }
+        int head_val = head->val;
+        head = head->next;
+        if (!isPalindromeHelper(head, len - 2))
+            return false;
+        head = head->next;
+        int tail_val = head->val;
+        return head_val == tail_val;
+    }
+};
+
+int main() {
+    Solution s;
+    ListNode l1(1);
+    ListNode l2(2);
+    ListNode l3(3);
+    ListNode l4(3);
+    ListNode l5(2);
+    ListNode l6(1);
+    l1.next = &l2;
+    cout << s.isPalindrome(&l1) << endl;
+    l2.next = &l3;
+    cout << s.isPalindrome(&l1) << endl;
+    l3.next = &l4;
+    cout << s.isPalindrome(&l1) << endl;
+    l4.next = &l5;
+    cout << s.isPalindrome(&l1) << endl;
+    l5.next = &l6;
+    cout << s.isPalindrome(&l1) << endl;
+    return 0;
+}
+