What is the time complexity of this code snippet? I am not sure of the calculation. This is in C#

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C#

algorithm

public static bool IsUnique(string inp)
{

Dictionary<char,int> dict = new Dictionary<char,int>();

foreach (char item in inp)
{
if (dict.ContainsKey(item))
return false;
else
dict.Add(item, 1);

}

return true;
}

What I have tried:

I have coded the above function in C# and the code is working fine. Can someone plz help me out with the time complexity of the above code snippet. Thanks in advance.