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I've used DP. IMO, the *real* problem is to cope with the requirement, that the leftmost pair of characters is always merged.

I couldn't get the obvious cubic algorithm to handle this. I got accepted with O(n^4) algo: for each morphing step, run the O(n^3) DP to find which pair of characters ...

Excuse me for saying a few words as the tasks all come from the contest jointly hosted by me and my friend.

Firstly, I would like to stress that we are hosting contest for interest and we are not doing any kind of business nor gaining any commercial benefits. According to the situation in Hong ...