Create linked-list-cycle-ii.js

LeetcodeProblems/linked-list-cycle-ii.js

@@ -0,0 +1,90 @@
+/*
+https://leetcode.com/problems/linked-list-cycle-ii/description/
+
+Given a linked list, return the node where the cycle begins. If there is no cycle, return null.
+
+Note: Do not modify the linked list.
+
+Follow up:
+Can you solve it without using extra space?
+*/
+
+
+// Optimal solution
+/**
+* @param {ListNode} head
+* @return {ListNode}
+*/
+var detectCycle = function(head) {
+  if (head === null)
+    return null;
+
+  var slow = head;
+  var fast = head;
+
+  while(fast.next !== null && fast.next.next !== null) {
+    slow = slow.next;
+    fast = fast.next.next;
+    if(fast == slow) {
+      var a = head;
+      var b = slow;
+      while(a !== b) {
+        a = a.next;
+        b = b.next;
+      }
+      return a;
+    }
+  }
+  return null;
+};
+
+// Naiver solution using a Set
+var detectCycle2 = function(head) {
+    if(head === null || head.next === null) {
+        return null;
+    }
+    var setNodes = new Set();
+    var iter = head;
+    while(iter !== null) {
+        if(setNodes.has(iter)) {
+            return iter;
+        }
+        setNodes.add(iter);
+        iter = iter.next
+    }
+    return null;
+};
+
+var main = function() {
+  const head = buildCycle();
+  console.log(detectCycle(head));
+}
+main();
+
+function ListNode(val) {
+  this.val = val;
+  this.next = null;
+}
+
+function buildCycle() {
+  var node1 =  new ListNode(1);
+  var node2 =  new ListNode(2);
+  var node3 =  new ListNode(3);
+  var node4 =  new ListNode(4);
+  var node5 =  new ListNode(5);
+
+  node1.next = node2;
+  node2.next = node3;
+  node3.next = node4;
+  node4.next = node5;
+  node5.next = node2; 
+
+  /*  1 -> 2 -> 3 -> 4 -> 5
+          \            /
+              - - - - -
+  */
+  return node1;
+}
+
+main();
+module.exports.main = main;