Tortoise and the hare

function findDuplicate(nums){
  let tortoise = nums[0]
  let hare = nums[0]
  while(true){
    tortoise = nums[tortoise]
    hare = nums[nums[hare]]
    if(tortoise == hare){
      break
    }
  }
  let ptr1 = nums[0]
  let ptr2 = tortoise
  while (ptr1 != ptr2){
    ptr1 = nums[ptr1]
    ptr2 = nums[ptr2]
  }
  return ptr1
}
console.log(findDuplicate([2,1,3,4,1]))