linked-list-cycle-ii.go

// https://leetcode.com/problems/linked-list-cycle-ii/ccccccccc

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func detectCycle(head *ListNode) *ListNode {
    if head == nil || head.Next == nil {
        return nil
    }
    
    hare := head
    tortoise := head
    
    for tortoise != nil && hare != nil && hare.Next != nil {
        tortoise = tortoise.Next // f(xi)
        hare = hare.Next.Next // f(f(xi))
        
        if hare == tortoise {
            break
        }
    }
    
    if hare != tortoise {
        return nil
    }
    
    tortoise = head
    
    for tortoise != hare {
        tortoise = tortoise.Next
        hare = hare.Next
    }
    
    return hare
}