Editorial for COCI '06 Contest 4 #4 Zbrka

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This problem is solved using dynamic programming. Let f(n, c) be the number of sequences of length n with confusion c.

The number of such sequences that start with the number 1 is f(n-1, c) because the 1 does not affect the confusion of the rest of the sequence and it makes no difference if we use numbers 1 \dots n-1 or 2 \dots n.

If 1 is the second number, then f(n, c) = f(n-1, c-1) because whichever element is first, it will form a confused pair with the 1. It's easy to see that the complete relation is:

\displaystyle f(n,c) = \sum_{i=0}^{n-1} f(n-1,c-i)

The time complexity of a direct implementation of this formula (using dynamic programming) would be \mathcal O(N^2 C), which is too slow.

We need to note that f(n, c) = f(n, c-1) + f(n-1, c) - f(n-1, c-n), which leads to a \mathcal O(NC) solution. It is also possible to cut down on the memory used by keeping only two rows of the matrix used for calculations at any time.


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