How does Interview Prep handle tortoise hare?

Interview Prep covers this with 2 copy-ready snippets on this page. The "Detect Cycle (Floyd's Tortoise & Hare)" snippet in Interview Prep uses `Use slow (1 step) and fast (2 steps) pointers.`.

Which code does the Interview Prep example use?

The "Detect Cycle (Floyd's Tortoise & Hare)" snippet uses `Use slow (1 step) and fast (2 steps) pointers.`, from the Linked Lists section of the Interview Prep cheat sheet.

What other Interview Prep snippets are shown for "tortoise hare"?

Besides "Detect Cycle (Floyd's Tortoise & Hare)", this page also shows "Pattern: Fast & Slow Pointer".

Tortoise hare

2 snippets in Interview Prep

Also written as tortoise and hare

DSAInterview Prep

Detect Cycle (Floyd's Tortoise & Hare)

DSA · Linked Lists

Syntax

Use slow (1 step) and fast (2 steps) pointers.
If they meet → cycle exists.
To find cycle start: reset one pointer to head, move both at 1 step.

Example

// JavaScript
function hasCycle(head) {
  let slow = head, fast = head;
  while (fast && fast.next) {
    slow = slow.next;
    fast = fast.next.next;
    if (slow === fast) return true;
  }
  return false;
}

function findCycleStart(head) {
  let slow = head, fast = head;
  while (fast && fast.next) {
    slow = slow.next;
    fast = fast.next.next;
    if (slow === fast) {
      slow = head;
      while (slow !== fast) {
        slow = slow.next;
        fast = fast.next;
      }
      return slow; // cycle start node
    }
  }
  return null;
}

# Python
def has_cycle(head):
    slow = fast = head
    while fast and fast.next:
        slow = slow.next
        fast = fast.next.next
        if slow is fast:
            return True
    return False

def find_cycle_start(head):
    slow = fast = head
    while fast and fast.next:
        slow = slow.next
        fast = fast.next.next
        if slow is fast:
            slow = head
            while slow is not fast:
                slow = slow.next
                fast = fast.next
            return slow
    return None

Output

Has cycle: True/False
Cycle start: node where cycle begins

Note Time O(n), Space O(1). The math behind finding the cycle start: when they meet, the distance from head to cycle start equals the distance from meeting point to cycle start (going around). Always use 'is' (identity) not '==' (equality) for node comparison.

Pattern: Fast & Slow Pointer

DSA · Common Patterns Summary

Syntax

When to use:
- Cycle detection (linked list, array)
- Finding middle of linked list
- Finding the start of a cycle
- Happy number problem

Slow moves 1 step, fast moves 2 steps.

Example

// JavaScript - Happy number (sum of squared digits eventually reaches 1)
function isHappy(n) {
  function digitSquareSum(num) {
    let sum = 0;
    while (num > 0) {
      const digit = num % 10;
      sum += digit * digit;
      num = Math.floor(num / 10);
    }
    return sum;
  }
  let slow = n, fast = n;
  do {
    slow = digitSquareSum(slow);
    fast = digitSquareSum(digitSquareSum(fast));
  } while (slow !== fast);
  return slow === 1;
}

# Python
def is_happy(n):
    def digit_square_sum(num):
        total = 0
        while num > 0:
            digit = num % 10
            total += digit * digit
            num //= 10
        return total
    slow = fast = n
    while True:
        slow = digit_square_sum(slow)
        fast = digit_square_sum(digit_square_sum(fast))
        if slow == fast:
            break
    return slow == 1

Output

is_happy(19) → True (19→82→68→100→1)
is_happy(2) → False (enters cycle)

Note The fast/slow pointer pattern detects cycles in any sequence where each value maps to the next. It uses O(1) space compared to a hash set. For linked lists, this is Floyd's algorithm. For number sequences (like happy number), the function that generates the next value plays the role of 'next pointer'.

Frequently asked questions

How does Interview Prep handle tortoise hare?: Interview Prep covers this with 2 copy-ready snippets on this page. The "Detect Cycle (Floyd's Tortoise & Hare)" snippet in Interview Prep uses `Use slow (1 step) and fast (2 steps) pointers.`.
Which code does the Interview Prep example use?: The "Detect Cycle (Floyd's Tortoise & Hare)" snippet uses `Use slow (1 step) and fast (2 steps) pointers.`, from the Linked Lists section of the Interview Prep cheat sheet.
What other Interview Prep snippets are shown for "tortoise hare"?: Besides "Detect Cycle (Floyd's Tortoise & Hare)", this page also shows "Pattern: Fast & Slow Pointer".
Is there anything to watch out for?: Yes. For "Detect Cycle (Floyd's Tortoise & Hare)": Time O(n), Space O(1). The math behind finding the cycle start: when they meet, the distance from head to cycle start equals the distance from meeting point to cycle start (going around). Always use 'is' (identity) not '==' (equality) for node comparison.