Create maximum-twin-sum-of-a-linked-list.cpp

Author: kamyu104

C++/maximum-twin-sum-of-a-linked-list.cpp

@@ -0,0 +1,44 @@
+// Time:  O(n)
+// Space: O(1)
+
+/**
+ * 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:
+    int pairSum(ListNode* head) {
+        ListNode dummy(0, head);
+        auto slow = dummy.next, fast = dummy.next;
+        while (fast->next && fast->next->next) {
+            slow = slow->next;
+            fast = fast->next->next;
+        }
+        int result = 0;
+        auto head2 = reverseList(slow);
+        while (head) {
+            result = max(result, head->val + head2->val);
+            head = head->next;
+            head2 = head2->next;
+        }
+        return result;
+    }
+
+private:
+    ListNode* reverseList(ListNode* head) {
+        ListNode dummy;
+        while (head) {
+            auto tmp = head->next;
+            head->next = dummy.next;
+            dummy.next = head;
+            head = tmp;
+        }
+        return dummy.next;
+    }
+};