求得所有包含在串s中而不包含在串t中的字符（s中重复的字符只选一个）构成的新串r，以及r中每个字符在s中第一次出现位置。-Obtain all included in the string s, not included in the character string t (s repeated characters Select only one) constituted new string r and r each character in s first position.