287. Find the Duplicate Number

class Solution:
    def findDuplicate(self, nums: List[int]) -> int:
        n=len(nums)
        s,f=nums[0],nums[nums[0]]
        while s!=f:
            s=nums[s]
            f=nums[nums[f]]
        #now make reset f to 0 and both s,f move only step. To meet the duplicate number
        f=0
        while s!=f:
            s,f=nums[s],nums[f]
        return s