add js soultion for leetcode 141 142 206

Author: swiftwind0405

README.md

@@ -226,7 +226,7 @@
 |209|[Minimum Size Subarray Sum](https://leetcode.com/problems/minimum-size-subarray-sum/)| |Medium|
 |208|[Implement Trie (Prefix Tree)](https://leetcode.com/problems/implement-trie-prefix-tree/)| |Medium|
 |207|[Course Schedule](https://leetcode.com/problems/course-schedule/)| |Medium|
-|206|[Reverse Linked List](https://leetcode.com/problems/reverse-linked-list/)| [java](./algorithms/reverseLinkedList/Solution.java) |Easy|
+|206|[Reverse Linked List](https://leetcode.com/problems/reverse-linked-list/)| [java](./algorithms/reverseLinkedList/Solution.java), [js](./algorithms/reverseLinkedList/reverseLinkedList.js) |Easy|
 |205|[Isomorphic Strings](https://leetcode.com/problems/isomorphic-strings/)| |Easy|
 |204|[Count Primes](https://leetcode.com/problems/count-primes/)| |Easy|
 |203|[Remove Linked List Elements](https://leetcode.com/problems/remove-linked-list-elements/)| [js](./algorithms/removeLinkedListElements/removeLinkedListElements.js),[java](./algorithms/removeLinkedListElements/Solution.java) |Easy|
@@ -284,8 +284,8 @@
 |145|[Binary Tree Postorder Traversal](https://leetcode.com/problems/binary-tree-postorder-traversal/)| [js](./algorithms/binaryTreePostorderTraversal/binaryTreePostorderTraversal.js) |Hard|
 |144|[Binary Tree Preorder Traversal](https://leetcode.com/problems/binary-tree-preorder-traversal/)| [js](./algorithms/binaryTreePreorderTraversal/binaryTreePreorderTraversal.js) |Medium|
 |143|[Reorder List](https://leetcode.com/problems/reorder-list/)| |Medium|
-|142|[Linked List Cycle II](https://leetcode.com/problems/linked-list-cycle-ii/)| |Medium|
-|141|[Linked List Cycle](https://leetcode.com/problems/linked-list-cycle/)| |Medium|
+|142|[Linked List Cycle II](https://leetcode.com/problems/linked-list-cycle-ii/)| [js](./algorithms/linkedListCycleII/linkedListCycleII.js) |Medium|
+|141|[Linked List Cycle](https://leetcode.com/problems/linked-list-cycle/)| [js](./algorithms/linkedListCycle/linkedListCycle.js) |Medium|
 |140|[Word Break II](https://leetcode.com/problems/word-break-ii/)| |Hard|
 |139|[Word Break](https://leetcode.com/problems/word-break/)| |Medium|
 |138|[Copy List with Random Pointer](https://leetcode.com/problems/copy-list-with-random-pointer/)| |Hard|

algorithms/linkedListCycle/linkedListCycle.js

@@ -0,0 +1,11 @@
+var hasCycle = function(head) {
+    if (!head || !head.next) return false;
+    let slow = head, fast = head.next;
+    while (fast) {
+        if (slow.val === fast.val) return true;
+        if (!fast.next || !fast.next.next) return false;
+        fast = fast.next.next;
+        slow = slow.next;
+    }
+    return false;
+};

algorithms/linkedListCycleII/linkedListCycleII.js

@@ -0,0 +1,24 @@
+var detectCycle = function(head) {
+    if (head === null) return null;
+    let slow = head, fast = head;
+    while (fast !== null) {
+        // 慢指针每次走 1步
+        slow = slow.next;
+        if (fast.next === null) return null;
+        // 快指针每次走2步
+        fast = fast.next.next;
+        // 快慢指针相遇
+        if (fast === slow) {
+            // 这时把快指针重新指向头部节点
+            let ptr = head;
+            // 当快慢指针相遇的时候，就是入环节点的位置
+            while (ptr !== slow) {
+                ptr = ptr.next;
+                slow = slow.next;
+            }
+            return ptr;
+        }
+
+    }
+    return null; 
+};

algorithms/reverseLinkedList/reverseLinkedList.js

@@ -0,0 +1,11 @@
+var reverseList = function(head) {
+    let prev = null;
+    let curr = head;
+    while(curr) {
+        let nextTemp = curr.next;
+        curr.next = prev;
+        prev = curr;
+        curr = nextTemp;
+    }
+    return prev;
+};