added Linked List Cycle II

Author: minhyeong-joe

Medium/LinkedListCycle2/README.md

@@ -0,0 +1,36 @@
+
+# Linked List Cycle II
+[Leetcode Link](https://leetcode.com/problems/linked-list-cycle-ii/)
+
+## Problem:
+
+Given a linked list, return the node where the cycle begins. If there is no cycle, return `null`.
+
+To represent a cycle in the given linked list, we use an integer `pos` which represents the position (0-indexed) in the linked list where tail connects to. If `pos` is `-1`, then there is no cycle in the linked list.
+
+Note: Do not modify the linked list.
+
+## Example:
+
+```
+Input: head = [3,2,0,-4], pos = 1
+Output: tail connects to node index 1
+Explanation: There is a cycle in the linked list, where tail connects to the second node.
+```
+![example](assets/example1.png)
+```
+Input: head = [1,2], pos = 0
+Output: tail connects to node index 0
+Explanation: There is a cycle in the linked list, where tail connects to the first node.
+```
+![example](assets/example2.png)
+```
+Input: head = [1], pos = -1
+Output: no cycle
+Explanation: There is no cycle in the linked list.
+```
+![example](assets/example3.png)
+
+## Note:
+
+Can you solve it without using extra space?

Medium/LinkedListCycle2/Solution.java

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+package Medium.LinkedListCycle2;
+
+// Definition for singly-linked list.
+class ListNode {
+ int val;
+ ListNode next;
+ ListNode(int x) {
+ val = x;
+ next = null;
+ }
+}
+
+/**
+ * Solution
+ * Use the hare and tortoise algorithm to detect cycle
+ */
+public class Solution {
+
+ public ListNode detectCycle(ListNode head) {
+ // empty list
+ if (head == null) {
+ return null;
+ }
+ ListNode hare = head;
+ ListNode tortoise = head;
+ do {
+ tortoise = tortoise.next;
+ // hare moves twice as fast as tortoise
+ hare = hare.next;
+ // if gets to the end of list (null), then no cycle
+ if (hare == null) {
+ return null;
+ }
+ hare = hare.next;
+ if (hare == null) {
+ return null;
+ }
+ } while (hare != tortoise);
+ // System.out.println("First met at (value): " + hare.val);
+ // return the tortoise to the beginning and move both hare and tortoise one step at a time
+ tortoise = head;
+ while (hare != tortoise) {
+ tortoise = tortoise.next;
+ hare = hare.next;
+ }
+ // System.out.println("Cycle starts at (value): " + tortoise.val);
+ // the second time they meet is the entry point of the cycle (return either tortoise or hare)
+ return tortoise;
+ }
+ 
+}

Medium/LinkedListCycle2/TestDriver.java

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+package Medium.LinkedListCycle2;
+
+import java.util.Arrays;
+
+public class TestDriver {
+ 
+ public static void main(String[] args) {
+ Solution sol = new Solution();
+ int i = 0;
+ 
+ int[] arr1 = {3,2,0,-4};
+ int pos1 = 1;
+ ListNode head1 = createLinkedList(arr1, pos1);
+ System.out.println("Input: head = " + Arrays.toString(arr1) + ", pos = " + pos1);
+ ListNode cycleEntry1 = sol.detectCycle(head1);
+ if (cycleEntry1 == null) {
+ System.out.println("Output: no cycle");
+ } else {
+ while (head1 != cycleEntry1) {
+ i++;
+ head1 = head1.next;
+ }
+ System.out.println("Output: tail connects to node index " + i);
+ }
+ System.out.println();
+ 
+ i = 0;
+ int[] arr2 = {1,2};
+ int pos2 = 0;
+ ListNode head2 = createLinkedList(arr2, pos2);
+ System.out.println("Input: head = " + Arrays.toString(arr2) + ", pos = " + pos2);
+ ListNode cycleEntry2 = sol.detectCycle(head2);
+ if (cycleEntry2 == null) {
+ System.out.println("Output: no cycle");
+ } else {
+ while (head2 != cycleEntry2) {
+ i++;
+ head2 = head2.next;
+ }
+ System.out.println("Output: tail connects to node index " + i);
+ }
+ System.out.println();
+ 
+ int[] arr3 = {1};
+ int pos3 = -1;
+ ListNode head3 = createLinkedList(arr3, pos3);
+ System.out.println("Input: head = " + Arrays.toString(arr3) + ", pos = " + pos3);
+ ListNode cycleEntry3 = sol.detectCycle(head3);
+ if (cycleEntry3 == null) {
+ System.out.println("Output: no cycle");
+ } else {
+ while (head3 != cycleEntry3) {
+ i++;
+ head3 = head3.next;
+ }
+ System.out.println("Output: tail connects to node index " + i);
+ }
+ System.out.println();
+ }
+
+ // helper to construct list from input array and pos
+ // returns the head node
+ public static ListNode createLinkedList(int[] arr, int pos) {
+ ListNode head = new ListNode(arr[0]);
+ ListNode temp = head;
+ for (int i = 1; i < arr.length; i++) { 
+ ListNode newNode = new ListNode(arr[i]);
+ temp.next = newNode;
+ temp = newNode;
+ }
+ if (pos >= 0) {
+ // find the node where cycle begins
+ ListNode iter = head;
+ for (int i = 0; i < pos; i++) {
+ iter = iter.next;
+ }
+ // temp is pointing to the last node in the list
+ temp.next = iter;
+ }
+ return head;
+ }
+}

Medium/LinkedListCycle2/assets/example1.png

@@ -50,6 +50,7 @@ Languages used: Java and Python
  - [Prison Cells After N Days](Medium/PrisonCellsAfterNDays)
  - [Minimum Number of Steps to Make Two Strings Anagram](Medium/MinStepsToMakeTwoStringsAnagram)
  - [Maximum Length of Pair Chain](Medium/MaximumLengthPairChain)
+ - [Linked List Cycle II](Medium/LinkedListCycle2)
 - Hard
 
 ---