Refine 206_Reverse_Linked_List

Author: qiyuangong

README.md

@@ -85,7 +85,7 @@ I'm currently working on [Analytics-Zoo](https://github.com/intel-analytics/anal
 | 198 | [House Robber](https://leetcode.com/problems/house-robber/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/198_House_Robber.py) | f(k) = max(f(k – 2) + num[k], f(k – 1)), O(n) and O(1) |
 | 200 | [Number of Islands](https://leetcode.com/problems/number-of-islands/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/200_Number_of_Islands.py) | 1. Quick union find, O(nlogn) and O(n^2)<br>2. BFS with marks, O(n^2) and O(1) |
 | 204 | [Count Primes](https://leetcode.com/problems/count-primes/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/204_Count_Primes.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/204_Count_Primes.java) [CPP](https://github.com/qiyuangong/leetcode/blob/master/cpp/204_Count_Primes.cpp) | [Sieve of Eratosthenes](https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes#Algorithm_complexity), O(nloglogn) and O(n) |
-| 206 | [Reverse Linked List](https://leetcode.com/problems/reverse-linked-list/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/206_Reverse_Linked_List.py) | 1. Stack, O(n) and O(n)<br>2. Traverse on prev and curr, then curr.next = prev, O(n) and O(1)<br>3. Recursion, O(n) and O(1) | 
+| 206 | [Reverse Linked List](https://leetcode.com/problems/reverse-linked-list/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/206_Reverse_Linked_List.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/206_Reverse_Linked_List.java) [CPP](https://github.com/qiyuangong/leetcode/blob/master/cpp/206_Reverse_Linked_List.cpp) | 1. Stack, O(n) and O(n)<br>2. Traverse on prev and curr, then curr.next = prev, O(n) and O(1)<br>3. Recursion, O(n) and O(1) |
 | 213 | [House Robber II](https://leetcode.com/problems/house-robber-ii/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/213_House_Robber_II.py) | f(k) = max(f(k – 2) + num[k], max(dp[0~ls-2],dp[1~ls-1], O(n) and O(1)|
 | 215 | [Kth Largest Element in an Array](https://leetcode.com/problems/kth-largest-element-in-an-array/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/215_Kth_Largest_Element_in_an_Array.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/215_Kth_Largest_Element_in_an_Array.java) | 1. Sort, O(n) and O(n)<br>2. Heap, O(nlgk) and O(n)<br>3. Quick selection, O(klgn) and O(n)|
 | 216 | [Combination Sum III](https://leetcode.com/problems/combination-sum-iii/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/216_Combination_Sum_III.py) | Generate all combinations of length k and keep those that sum to n|

cpp/206_Reverse_Linked_List.cpp

@@ -8,19 +8,23 @@
  */
 class Solution {
 public:
-    ListNode* reverseList(ListNode* head) {
-        if(head==NULL)return NULL;
-        if(head->next==NULL) return head;
-        ListNode* p=head;
-        ListNode* c=head->next;
-        head->next=NULL;
-        while(c->next){
-            ListNode* n = c->next;
-            c->next=p;
-            p=c;
-            c=n;
+    ListNode *reverseList(ListNode *head) {
+        if (head == NULL)
+            return NULL;
+        if (head->next == NULL)
+            return head;
+        // Previous pointer
+        ListNode *previous = head;
+        // Current pointer
+        ListNode *curr = head->next;
+        head->next = NULL;
+        while (curr->next) {
+            ListNode *next = curr->next;
+            curr->next = previous;
+            previous = curr;
+            curr = next;
         }
-        c->next=p;
-        return c;
+        curr->next = previous;
+        return curr;
     }
 };

java/206_Reverse_Linked_List.java

@@ -0,0 +1,38 @@
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode() {}
+ *     ListNode(int val) { this.val = val; }
+ *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
+ * }
+ */
+class Solution {
+    // https://leetcode.com/problems/reverse-linked-list/discuss/58125/In-place-iterative-and-recursive-Java-solution
+    public ListNode reverseList(ListNode head) {
+        // Iterative solution
+        ListNode newHead = null;
+        while (head != null) {
+            ListNode next = head.next;
+            head.next = newHead;
+            newHead = head;
+            head = next;
+        }
+        return newHead;
+    }
+    
+    // Recursive
+    /*
+    public ListNode reverseList(ListNode head) {
+        return reverseListInt(head, null);
+    }
+    
+    private ListNode reverseListInt(ListNode head, ListNode newHead) {
+        if (head == null)
+            return newHead;
+        ListNode next = head.next;
+        head.next = newHead;
+        return reverseListInt(next, head);
+    }*/
+}