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Author: BaffinLee

201-300/206. Reverse Linked List.md

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+# 206. Reverse Linked List
+
+- Difficulty: Easy.
+- Related Topics: Linked List.
+- Similar Questions: Reverse Linked List II, Binary Tree Upside Down, Palindrome Linked List.
+
+## Problem
+
+Reverse a singly linked list.
+
+**Example:**
+
+```
+Input: 1->2->3->4->5->NULL
+Output: 5->4->3->2->1->NULL
+```
+
+**Follow up:**
+
+A linked list can be reversed either iteratively or recursively. Could you implement both?
+
+## Solution 1
+
+```javascript
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val) {
+ *     this.val = val;
+ *     this.next = null;
+ * }
+ */
+/**
+ * @param {ListNode} head
+ * @return {ListNode}
+ */
+var reverseList = function(head) {
+  var newHead = null;
+  var tmp = null;
+  while (head) {
+    tmp = head.next;
+    head.next = newHead;
+    newHead = head;
+    head = tmp;
+  }
+  return newHead;
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(n).
+* Space complexity : O(1).
+
+## Solution 2
+
+```javascript
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val) {
+ *     this.val = val;
+ *     this.next = null;
+ * }
+ */
+/**
+ * @param {ListNode} head
+ * @return {ListNode}
+ */
+var reverseList = function(head) {
+  return reverse(null, head);
+};
+
+var reverse = function (newHead, head) {
+  if (!head) return newHead;
+  var tmp = head.next;
+  head.next = newHead;
+  return reverse(head, tmp);
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(n).
+* Space complexity : O(n).