public class Solution {    public int FindMaximumXOR(int[] nums) {        int max = 0, mask = 0;            for (int i = 31; i >= 0; i--)            {                mask = mask | (1 << i);                HashSet
     
       set = new HashSet
      
       ();                foreach (int num in nums)                {                    set.Add(num & mask);                }                int tmp = max | (1 << i);                foreach (int prefix in set)                {                    if (set.Contains(tmp ^ prefix))                    {                        max = tmp;                        break;                    }                }            }            return max;    }}