Problem

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

Example 3:

Input: nums = [3,3,3,3,3] 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?

Pre analysis

I can use set to keep track of the numbers and check if the number is already in the set. If it is, return the number.