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Author: whd

234 Palindrome Linked List .cpp

@@ -0,0 +1,55 @@
+/**
+ * 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) {
+        if (!head) {
+            return true;
+        }
+        auto getLen = [] (ListNode *x) {
+            int s = 0;
+            while (x) {
+                s++;
+                x = x->next;
+            }
+            return s;
+        };
+        int len = getLen(head);
+        auto get = [] (ListNode *x, int k) {
+            while (k--) {
+                x = x->next;
+            }
+            return x;
+        };
+        if (len == 1) {
+            return true;
+        }
+        ListNode *h1 = head, *h2 = get(head, (len + 1) / 2 - 1);
+        auto next = h2->next;
+        h2->next = nullptr;
+        h2 = next;
+        auto reversed = [] (ListNode *x) {
+            ListNode *newHead = nullptr;
+            while (x) {
+                auto next = x->next;
+                x->next = newHead;
+                newHead = x;
+                x = next;
+            }
+            return newHead;
+        };
+        h2 = reversed(h2);
+        for (; h1 && h2; h1 = h1->next, h2 = h2->next) {
+            if (h1->val != h2->val) {
+                return false;
+            }
+        }
+        return true;
+    }
+};