LeetCode: Linked List Cycle
Performance

You have implemented Floyd’s Cycle-Finding Algorithm which adheres to \$0(1)\$ storage space. An alternative exists Brent’s Cycle Detection Algorithm which uses the same storage space. Check out this review on Computer Science SE for a comparison. It appears in general, Brent's algorithm is faster.


  According to Brent's paper, the complexity of Floyd's algorithm is between 3max(m,n) and 3(m+n), and that of Brent's is at most
  2max(m,n)+n, which is always better than 3max(m,n).


courtesy of Yuval Filmus' answer at CS

Style Guidelines


use var to declare a variable, specially when the type can be inferred from code
use a separate line for declaring attributes on top of members
use a white space after a method name and the opening parenthesis
use a white space after the while statement
use white space around operators (!=)
remove redundant nested else branches if the if branch always performs a return