Find First Non-Repeated Character in a String

This doesn't look difficult right :) however can u think of different ways to find the first non-repeated character in a string?

I tried 3 different ways in C#. One with O(n^2) and 2 ways with O(n). Please suggest if you can think of other better ways :)

///Complexity: O(n^2)

public static char FirstNonRepeatedCharInString (string str)
{
    int i, j;
    bool isRepeted = false;
    char[] chars = str.ToCharArray();
    for (i=0; i<chars.Length;i++)
    {
        isRepeted = false;
        for (j=0;j<chars.Length;j++)
        {
            if ((i!=j) && (chars[i]==chars[j]))
            {
               isRepeted = true;
               break;
             }
         }
         if (isRepeted ==false)
         {
            return chars[i];
          }
      }
      return ' ';
   }
---------------------------------------------------------------------------------------------
///Complexity: O(n)
public static char FirstNonRepeatedCharInString2(string str)
{
    Dictionary<char, int> l1 = new Dictionary<char, int>();
    int temp = 0;
    foreach (char c in str.ToCharArray())
    {
        if (l1.Keys.Contains(c))
        {
            temp = l1[c];
            l1[c] = temp + 1;
            continue;
        }
        l1.Add(c,1);
      }
     if (l1.Values.Contains(1))
     {
         return l1.First(x => x.Value == 1).Key;
     }
     return ' ';
 }

----------------------------------------------------------------------------------------------

///Complexity: O(256) + O(n) + O(n) = O(n)

public static char FirstNonRepeatedCharInString3(string str)
{
   int[] chars = new int[256];
   for (int i = 0; i < chars.Length; i++)
   {
      chars[i] = 0;
   }

   for (int i = 0; i < str.Length; i++)
   {
      chars[str[i]]++;
   }
   for (int i = 0; i < str.Length; i++)
   {
      if (chars[str[i]] == 1)
      {
         return str[i];
      }
   }
   return ' ';
}