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{{Problem C :- Barcode}}{{Codeforces Round 139 DIV 2}}
By sameer_hack, history, 8 years ago, In English

I am trying to solve this problem and I am not able to understand the editorial that why
dp[0][i]=min(dp[1][i-a]+sum-of-whites(i-a+1,i))
dp[1][i]=min(dp[0][i-a]+sum-of-blacks(i-a+1,i))
where a belongs to [x,y]
Editorial Link :- Editorial
Question Link :- Question

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8 years ago, hide # |
 
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Can anybody from community can help with me this question ? Its been 7hrs and I dont know why people doesnt respond to question that may look small to you but its like mount Everest for me. :/

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    8 years ago, hide # ^ |
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    dp[c][i] — minimal cost of painting the first i columns in NxM rectangle into a barcode the way its last stripe will be (c == 0 ? black : white).

    dp[0][i]=min(dp[1][i-a]+sum-of-whites(i-a+1,i))

    This line checks if it is optimal to paint last a lines in black.

    dp[1][i]=min(dp[0][i-a]+sum-of-blacks(i-a+1,i))

    And this line checks if it is optimal to paint last a lines in white.

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      8 years ago, hide # ^ |
       
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      Can you please explain how the two recurrence relation is formed ?

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        8 years ago, hide # ^ |
         
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        Well, let's look on Nxi rectangle formed with the first i columns. What can we say about its last stripe? Either it's black or white and his width is ranged from x to y. Therefore, we handle 2 * (y — x + 1) variants. Let's imagine the last stripe in your subrectangle is white. It means that previous stripe's color has to be the different (black). Then, for each a x <= a <= y check the cost of oainting rectangle Nx(i-a) in stripes so it will end with the black stripe (dp[0][i-a])and add the cost of painting columns from i-a+1 to i in white (sum-of-whites(i-a+1,i)). Then we keep the best outcome.

        Same thing with the case, where the last stripe in rectangle Nxi is white, but with reverse colouring.

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