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Why is the 5th test case in "B. Flip the Bits" of "Codeforces Round #712 (Div. 2)" is impossible?
Автор vrintle, история, 4 года назад, По-английски

Hello everyone!

Here is the link to the problem: https://codeforces.net/contest/1504/problem/B

In the 5th test case, $$$A$$$ can be converted to $$$B$$$ as:

000111 -> [000111] -> 111000
111000 -> 11[10]00 -> 110100

As, both the prefix contains equal $$$1$$$ and $$$0$$$, so I think this is correct. But, the answer says, it is impossible to convert $$$A$$$ to $$$B$$$. Why?

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4 года назад, # |
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read the question again

you can select any prefix of a with an equal number of 0 and 1 symbols

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    4 года назад, # ^ |
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    Oh, I just realized that prefix should start at position 1, right?

    But, I think it should be made clear in the problem statement, what a prefix is, like they do when the problem is related to subarray, substring or subsequences, which are more often terms than prefix.

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