①：用字典存储（内存消耗大）

public class Solution {
    public bool ContainsDuplicate(int[] nums) {
      var dict = new Dictionary<int, int>();
            for (int i = 0; i < nums.Length; i++) 
            {
                if (dict.ContainsKey(nums[i])) 
                {
                    return true;
                }
                dict.Add(nums[i], 1);
            }
            return false;
    }
}

②：排序（消耗也都挺大的）

public class Solution {
    public bool ContainsDuplicate(int[] nums) {
         List<int> list = new List<int>();
            list = nums.ToList();
            list.Sort();
            for (int i = 0; i < list.Count-1; i++) 
            {
                if (list[i] == list[i + 1]) 
                {
                    return true;
                }
            }
            return false;
    }
}