Custom String Sort

You are given two strings: 'order' and 'str'. The 'order' string contains a collection of unique characters which determines the priority of the characters in 'str'. Your task is to sort 'str' such that the characters appear in the order they are found in 'order'. Characters not in 'order' should be appended at the end in their original relative order. If 'order' is empty, return 'str' as is. Example Input: 'cba' 'abdc' Example Output: 'dcbab'