Use the least code to find the first character in a string that does not repeat (that is, only appears once)

I saw a post on csdn:
Interview Questions:
Use the least code to find the first character in a string that does not repeat (that is, only appears once)
That is, string AB = "awbbbccdefafgg ";
The result is W.

You have implemented the following using the conventional method:
String AB = "awbbbccdefafgg ";
Int tempequal = 0
String tempvalue = "";
If (AB! = "")
{
Char [] A = AB. tochararray ();
For (INT I = 0; I <AB. length; I ++) // traverses the Array
{
For (Int J = 0; j <A. length; j ++) // traverse the array again
{
If (AB [I] = A [J]) // character-by-character comparison
{
Tempequal ++;
}

}
If (tempequal = 1)
{
Tempvalue = AB [I]. tostring ();
Break;
}
Tempequal = 0;
}

Response. Write ("value:" + tempvalue );
}

But I feel that this code is long and I don't know how to implement it easily.