xxxxxxxxxx
def find_duplicates(nums):
tortoise = nums[0]
hare = nums[0]
while True:
tortoise = nums[tortoise]
hare = nums[hare]
if tortoise == hare:
break
ptr1 = nums[0]
ptr2 = tortoise
while ptr1 != ptr2:
ptr1 = nums[ptr1]
ptr2 = nums[ptr2]
return ptr1
print(find_duplicates([1, 2, 3, 3, 4, 5, 6]))