Hardest

I just did a phone screen with Chime and received the hardest coding question I’ve ever seen. Idk if I’m mentally blocked here or this is straight ridiculous. The question was: You are given a string representing numbers from 1 to n. These numbers are not in order. Find the missing number. Eg: N = 10, s = 1098253471 Ans = 6 I had 30 minutes to solve. This gets really hard when n is double or triple digits, you don’t know what digit belongs to what number so you have to test all possibilities. Is there any way to do this without just checking every possibility and marking off the digits you used as you go? Failed btw.