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🐢🐇 Fast and Slow Pointers in Java – The Ultimate Guide

The Fast and Slow Pointer technique is a powerful approach used to solve problems involving linked lists, cycles, palindromes, and more.

Also known as Floyd's Cycle Detection Algorithm, this technique is a must-have in your interview toolkit.

📘 What is Fast and Slow Pointer?

Two pointers are used:

  • Slow Pointer (Tortoise) – moves 1 step at a time
  • Fast Pointer (Hare) – moves 2 steps at a time

When both pointers are used on a data structure (like a linked list), their interaction can help detect cycles, find the middle, check for palindromes, and more.

🧠 Why Use This Technique?

✅ Detect cycle in linked list

✅ Find starting point of the cycle

✅ Find middle of linked list

✅ Check for palindrome

✅ Detect intersection in linked lists

🧩 Basic Template 

ListNode slow = head;
ListNode fast = head;

while (fast != null && fast.next != null) {
    slow = slow.next;
    fast = fast.next.next;

    if (slow == fast) {
        // Cycle detected
    }
}
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🔨 Java Code Examples

📘 1. Detect Cycle in a Linked List 

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;
}
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📌 If they meet, a cycle exists.

⏱ Time: O(n), 🧠 Space: O(1)

📘 2. Find Start of Cycle 

public ListNode detectCycle(ListNode head) {
    ListNode slow = head;
    ListNode fast = head;

    while (fast != null && fast.next != null) {
        slow = slow.next;
        fast = fast.next.next;

        if (slow == fast) {
            ListNode entry = head;
            while (entry != slow) {
                entry = entry.next;
                slow = slow.next;
            }
            return entry;
        }
    }
    return null;
}
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📌 After detection, move one pointer to head and move both at 1 step.

They’ll meet at cycle start.

📘 3. Find Middle of Linked List 

public ListNode findMiddle(ListNode head) {
    ListNode slow = head;
    ListNode fast = head;

    while (fast != null && fast.next != null) {
        slow = slow.next;
        fast = fast.next.next;
    }

    return slow; // Middle node
}
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📌 If even length, returns second middle.

⏱ Time: O(n)

📘 4. Check if Linked List is Palindrome 

public boolean isPalindrome(ListNode head) {
    ListNode slow = head;
    ListNode fast = head;

    // Find middle
    while (fast != null && fast.next != null) {
        slow = slow.next;
        fast = fast.next.next;
    }

    // Reverse second half
    ListNode secondHalf = reverse(slow);
    ListNode firstHalf = head;

    while (secondHalf != null) {
        if (secondHalf.val != firstHalf.val) return false;
        secondHalf = secondHalf.next;
        firstHalf = firstHalf.next;
    }
    return true;
}

private ListNode reverse(ListNode head) {
    ListNode prev = null;
    while (head != null) {
        ListNode next = head.next;
        head.next = prev;
        prev = head;
        head = next;
    }
    return prev;
}
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📌 Uses fast/slow to split list, then reverse & compare.

⏱ Time: O(n), Space: O(1)

📘 5. Happy Number Problem (LeetCode 202) 

public boolean isHappy(int n) {
    int slow = n;
    int fast = getSquare(n);

    while (slow != fast) {
        slow = getSquare(slow);
        fast = getSquare(getSquare(fast));
    }

    return slow == 1;
}

private int getSquare(int n) {
    int sum = 0;
    while (n > 0) {
        int digit = n % 10;
        sum += digit * digit;
        n /= 10;
    }
    return sum;
}
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📌 Fast and slow applied on numbers instead of linked lists.

Detect cycle in digit squares.

📊 Complexity

Problem Time Space
Detect cycle O(n) O(1)
Cycle start O(n) O(1)
Find middle O(n) O(1)
Palindrome O(n) O(1)
Happy number O(log n) O(1)

💡 Interview Questions

Problem Platform
Linked List Cycle I & II LeetCode 141, 142
Find Middle of Linked List GFG
Palindrome Linked List LeetCode 234
Intersection of Two Linked Lists LeetCode 160
Happy Number LeetCode 202

🎯 Final Thoughts

Fast and slow pointers are deceptively simple but incredibly powerful. They let you optimize your linked list solutions without extra space and are a must-know for technical interviews.

Master this pattern, and you'll unlock a whole set of problems with clean, efficient solutions. 🚀

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