217

Well, simple. Use hashset.

Time - $O(n)$. Space - $O(n)$.

public class Solution {
    public bool ContainsDuplicate(int[] nums) {
        var seen = new HashSet<int>();
 
        foreach (int n in nums)
        {
            if (seen.Contains(n))
                return true;
 
            seen.Add(n);
        }
 
        return false;
    }
}

---

Other approaches?

Sorting + Adjacent Check,
Basically, sort the array. If duplicates exist, they will be adjacent.

Time - $O(nlogn)$. Space - $O(1)$ (if in-place sort).

public class Solution
{
    public bool ContainsDuplicate(int[] nums)
    {
        Array.Sort(nums);
 
        for (int i = 1; i < nums.Length; i++)
        {
            if (nums[i] == nums[i - 1])
                return true;
        }
 
        return false;
    }
}

---

Another hashset pattern,
HashSet One-Liner Trick (Size Comparison)

public class Solution
{
    public bool ContainsDuplicate(int[] nums)
    {
        return nums.Length != new HashSet<int>(nums).Count;
    }
}

This works because a HashSet only keeps unique elements, so size reduces if duplicates exist.