287. Find the Duplicate Number

Medium

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Given an array of integers nums containing n + 1 integers where each integer is in the range [1, n] inclusive.

There is only one repeated number in nums, return this repeated number.

You must solve the problem without modifying the array nums and uses only constant extra space.

Example 1:

Input: nums = [1,3,4,2,2]
Output: 2

Example 2:

Input: nums = [3,1,3,4,2]
Output: 3

Constraints:

• 1 <= n <= 105

• nums.length == n + 1

• 1 <= nums[i] <= n

• All the integers in nums appear only once except for precisely one integer which appears two or more times.

Follow up:

• How can we prove that at least one duplicate number must exist in nums?

• Can you solve the problem in linear runtime complexity?

Same as Problem 142 : Linked List Cycle 2

For List : [6, 2, 4, 1, 3, 2, 5, 2] below cycle will get created

class Solution:
    def findDuplicate(self, nums: List[int]) -> int:
        slow = fast = nums[0]
        while True:
            slow = nums[slow]
            fast = nums[nums[fast]]
            if slow == fast:
                slow = nums[0]
                while slow != fast:
                    slow = nums[slow]
                    fast = nums[fast]
                return fast