4 rounds of on-site technical interview, first three are just questions with solution patterns on leetcode, 2 medium and 1 easy. What I want to highlight and share here is the 4th round, which breaks my confidence.

Question:
You're giving two strings source and target, write a function to determine if we could convert source to target.

Limits:

• Both source and target are consisted with lower letters, which is a-z.
• All available letters in converting are a-z, which means you can not convert one letter to 1 or # or others.

Converting rules:

• In one convert operation, you can replace one value with another value, which means if there're three letters with same value, you need to replace them together. For example, I have "abca", now I can replace "a" to "d", then it become "dbcd". "dbca" is impossible!
• You can do convert operation as many as you want.

How I feel:
I don't know if I described clearly about the question, I've struggled about 20 minutes with the interviewer to clearify the question by talking (Now I realized that just asking the interviewer to paste the question is more efficient!). Then all I have was only 20 minutes (Because bath break, self introduction and environment setting cost 5 mins) to figure out the solution. It's really hard to me, I'm thinking if there is a pattern to handle this but finally with interviewer's hint (almost equals telling me the answer), I found it's a un-pattern question and all it's testing is how I think, not coding.
I would say that I performed well in all other sessions, but get bad feed back from this session, and finally rejected by the hiring committee.
I'm still in practicing, but I know the next time I meet questions like this, I can still not figure out the solution, but maybe I could perform better on communication and problem solving pattern(figure out all cases and write them down then try to solve one by one) to get the luck to earn a neutral feedback.

Solutions:

1. Check corner cases, like null.
2. Check the length.
3. Check violations like "#".
4. Check if there exists mapping of one to multiple, like "aba"->"cde" where "a" maps to "c" and "e". If so, return false.
5. Check if all letters are used in target, if not then return true. Because we could use the un-used letter to convert each value one by one without affecting others.

Below are some examples:

Question: "ab"->"cd"
Answer: true
Solution: convert "a" to "c" then "b" to "d", "ab"->"cb"->"cd"

Question: "aba"->"cde"
Answer: false
Solution: the first "a" and last "a" are always same, it's impossible to make them one is "c" and the other is "e"

Question: "ab"->"ba"
Answer: true
Solution: convert "a" to "d", then "b" to "a", then "d" to "b", "ab"->"db"->"da"->"ba"