Tranverse the string , fills the associative array count [ ] and count number of unique characters
for i in range ( n ) : NEW_LINE INDENT if count [ ord ( s [ i ] ) - ord ( ' a ' ) ] == 0 : NEW_LINE INDENT u += 1 NEW_LINE DEDENT count [ ord ( s [ i ] ) - ord ( ' a ' ) ] += 1 NEW_LINE DEDENT