https://leetcode.com/problems/find-the-duplicate-number/

leet_code_find_the_duplicate_number.cs

public class Solution
{
    public int FindDuplicate(int[] nums)
    {
        var i1 = nums[0];
        var i2 = nums[0];
        
        do
        {
            i1 = nums[i1];
            i2 = nums[nums[i2]];
        }
        while (i1 != i2);
        
        i2 = nums[0];
        
        while (i1 != i2)
        {
            i1 = nums[i1];
            i2 = nums[i2];
        }
        
        return i1;
    }
}