Tortoise and Hare Algorithm

Used for finding cycles in linked list in $O (n)$
Uses two pointers, $s l o w$ and $f a s t$ , pointing at head initially
$s l o w$ moves one step, $f a s t$ moves two steps at a time
Check if at any point both pointers point to same node
Reset $s l o w$ back to head of linked list
Move both pointers one step at a time
The point they meet at is starting point of cycle

# links
links = [1, 2, 3, 4, 5, 6, 2]
 
# list head
head = 0
 
slow = links[head]
fast = links[links[head]]
 
while slow != fast:
    # move slow once, fast twice
    slow = links[slow]
    fast = links[links[fast]]
 
# reset slow to head
slow = head
 
while slow != fast:
    # move both once
    slow = links[slow]
    fast = links[fast]
 
print(slow)