Question

141. Linked List Cycle

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Description

Given head, the head of a linked list, determine if the linked list has a cycle in it.

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail's next pointer is connected to. Note that pos is not passed as a parameter.

Return true_ if there is a cycle in the linked list_. Otherwise, return false.

 

Example 1:

Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where the tail connects to the 1st node (0-indexed).

Example 2:

Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where the tail connects to the 0th node.

Example 3:

Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.

 

Constraints:

• The number of the nodes in the list is in the range [0, 104].
• -105 <= Node.val <= 105
• pos is -1 or a valid index in the linked-list.

 

Follow up: Can you solve it using O(1) (i.e. constant) memory?

Solutions

Solution 1

# Definition for singly-linked list.
# class ListNode:
#   def __init__(self, x):
#     self.val = x
#     self.next = None


class Solution:
  def hasCycle(self, head: ListNode) -> bool:
    slow = fast = head
    while fast and fast.next:
      slow, fast = slow.next, fast.next.next
      if slow == fast:
        return True
    return False

/**
 * Definition for singly-linked list.
 * class ListNode {
 *   int val;
 *   ListNode next;
 *   ListNode(int x) {
 *     val = x;
 *     next = null;
 *   }
 * }
 */
public class Solution {
  public boolean hasCycle(ListNode head) {
    ListNode slow = head;
    ListNode fast = head;
    while (fast != null && fast.next != null) {
      slow = slow.next;
      fast = fast.next.next;
      if (slow == fast) {
        return true;
      }
    }
    return false;
  }
}

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *   int val;
 *   ListNode *next;
 *   ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
  bool hasCycle(ListNode* head) {
    ListNode* slow = head;
    ListNode* fast = head;
    while (fast && fast->next) {
      slow = slow->next;
      fast = fast->next->next;
      if (slow == fast) {
        return true;
      }
    }
    return false;
  }
};

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *   Val int
 *   Next *ListNode
 * }
 */
func hasCycle(head *ListNode) bool {
  slow, fast := head, head
  for fast != nil && fast.Next != nil {
    slow, fast = slow.Next, fast.Next.Next
    if slow == fast {
      return true
    }
  }
  return false
}

/**
 * Definition for singly-linked list.
 * class ListNode {
 *   val: number
 *   next: ListNode | null
 *   constructor(val?: number, next?: ListNode | null) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 *   }
 * }
 */

function hasCycle(head: ListNode | null): boolean {
  const set = new Set<ListNode>();
  let node = head;
  while (node !== null) {
    if (set.has(node)) {
      return true;
    }
    set.add(node);
    node = node.next;
  }
  return false;
}

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *   this.val = val;
 *   this.next = null;
 * }
 */

/**
 * @param {ListNode} head
 * @return {boolean}
 */
var hasCycle = function (head) {
  let slow = head;
  let fast = head;
  while (fast && fast.next) {
    slow = slow.next;
    fast = fast.next.next;
    if (slow === fast) {
      return true;
    }
  }
  return false;
};

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *   public int val;
 *   public ListNode next;
 *   public ListNode(int x) {
 *     val = x;
 *     next = null;
 *   }
 * }
 */
public class Solution {
  public bool HasCycle(ListNode head) {
    var fast = head;
    var slow = head;
    while (fast != null && fast.next != null) {
      fast = fast.next.next;
      slow = slow.next;
      if (fast == slow) {
        return true;
      }
    }
    return false;
  }
}

Solution 2

/**
 * Definition for singly-linked list.
 * class ListNode {
 *   val: number
 *   next: ListNode | null
 *   constructor(val?: number, next?: ListNode | null) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 *   }
 * }
 */

function hasCycle(head: ListNode | null): boolean {
  let slow = head;
  let fast = head;
  while (fast !== null && fast.next !== null) {
    slow = slow.next;
    fast = fast.next.next;
    if (slow === fast) {
      return true;
    }
  }
  return false;
}