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Linked List Cycle Detection

Certification: Basic Level Accuracy: 0% Submissions: 0 Points: 5

Write a C program to detect if a linked list has a cycle in it. A cycle in a linked list occurs when a node's next pointer points to a node that was previously visited in the list, creating a loop. The program should return true if there is a cycle in the linked list, otherwise, return false.

Example 1
    
  
  • Input: head = [3,2,0,-4], pos = 1
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  • Output: true
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  • Explanation: 
    
      
      
    • Step 1: The linked list has nodes with values 3, 2, 0, and -4.
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    • Step 2: The last node points back to the second node (position 1).
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    • Step 3: This creates a cycle in the linked list.
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    • Step 4: Therefore, the output is true.
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Example 2
    
  
  • Input: head = [1,2], pos = 0
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  • Output: true
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  • Explanation: 
    
      
      
    • Step 1: The linked list has nodes with values 1 and 2.
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    • Step 2: The last node points back to the first node (position 0).
      • 
      
    • Step 3: This creates a cycle in the linked list.
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    • Step 4: Therefore, the output is true.
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Example 3
    
  
  • Input: head = [1], pos = -1
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  • Output: false
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  • Explanation: 
    
      
      
    • Step 1: The linked list has only one node with value 1.
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    • Step 2: There is no next node, so no cycle can exist.
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    • Step 3: Therefore, the output is false.
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Constraints
    
  
  • The number of nodes in the list is in the range [0, 10^4]
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  • -10^5 <= Node.val <= 10^5
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  • pos is -1 or a valid index in the linked-list
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  • Time Complexity: O(n) where n is the number of nodes in the linked list
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  • Space Complexity: O(1)
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Hint
Solution

Solution Hints

    
  
  • A naive approach would be to use a hash set to track visited nodes, but that would require O(n) space
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  • Consider using the "tortoise and hare" algorithm (Floyd's Cycle-Finding Algorithm)
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  • Use two pointers that traverse the list at different speeds
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  • If there is a cycle, the faster pointer will eventually catch up to the slower one
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  • If there's no cycle, the faster pointer will reach the end of the list
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Steps to solve by this approach:

 Step 1: Check if the list is empty or has only one node. If so, return false as no cycle can exist.

 Step 2: Initialize two pointers: 'slow' and 'fast', both starting at the head of the list.

 Step 3: Move the 'slow' pointer one step at a time and the 'fast' pointer two steps at a time.

 Step 4: If there's a cycle, the 'fast' pointer will eventually meet the 'slow' pointer inside the cycle.

 Step 5: If the 'fast' pointer reaches NULL or a node whose next is NULL, then there's no cycle.

 Step 6: Return true if the pointers meet, indicating a cycle was detected.

 Step 7: Return false if the fast pointer reaches the end, indicating no cycle exists.

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