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🐢🐇 Fast and Slow Pointer – Java Cheat Sheet (One Page)

📌 What Is It?

Two pointers move at different speeds. Often used for cycle detection, midpoint finding, and palindrome logic.

• Slow moves one step at a time
• Fast moves two steps at a time

🧱 Pattern Template

ListNode slow = head;
ListNode fast = head;

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

✅ Use Cases

• Detect cycles in linked list or array
• Find middle of a linked list
• Check if a linked list is a palindrome
• Detect infinite loop (e.g., Happy Number)
• Find circular paths in arrays

📘 Common LeetCode Problems

• 141: Linked List Cycle [Easy] Fast & Slow Pointer
• 876: Middle of the Linked List [Easy] Fast & Slow Pointer
• 234: Palindrome Linked List [Medium] Fast & Slow Pointer, Two Pointers
• 202: Happy Number [Easy] Fast & Slow Pointer, Math
• 457: Circular Array Loop [Medium] Fast & Slow Pointer, Array

🧪 Example 1: Detect Cycle in Linked List

public boolean hasCycle(ListNode head) {
    ListNode slow = head, fast = head;
    while (fast != null && fast.next != null) {
        slow = slow.next;
        fast = fast.next.next;
        if (slow == fast) return true;
    }
    return false;
}

▶️ View Animation

🧪 Example 2: Find Middle of Linked List

public ListNode middleNode(ListNode head) {
    ListNode slow = head, fast = head;
    while (fast != null && fast.next != null) {
        slow = slow.next;
        fast = fast.next.next;
    }
    return slow;
}

▶️ View Animation

💡 Pro Tips

• Fast pointer catches slow → cycle exists
• Slow can be used to reverse second half for palindrome check
• Works for arrays using indices too