Solution: Reverse Linked List

Author: obzva

reverse-linked-list/flynn.cpp

@@ -0,0 +1,54 @@
+/**
+ * 풀이
+ * - 생략, 주석 참고
+ * 
+ * Big O
+ * - N: 주어진 링크드 리스트 `head`의 길이
+ * 
+ * - Time complexity: O(N)
+ * - Space complexity: 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:
+    ListNode* reverseList(ListNode* head) {
+        if (head == nullptr) return head;
+        // example: a - b - c - d
+
+        ListNode* root = new ListNode();
+        root->next = head;
+        // root - a - b - c - d
+
+        ListNode* p = head;
+        ListNode* q = p->next;
+        // root - a - b - c - d
+        //       (p) (q)
+
+        while (q != nullptr) {
+            p->next = q->next;
+            // root - a - c - d
+            //        b /
+            q->next = root->next;
+            // root - a - c - d
+            //    b /
+            root->next = q;
+            // root - b - a - c - d
+            //       (q) (p)
+            q = p->next;
+            // root - b - a - c - d
+            //           (p) (q)
+        }
+
+        return root->next;
+    }
+};