https://www.hackerrank.com/challenges/tutzki-and-lcs/problem
I ve been trying to solve this for hours and could'nt find any understandable online solution, Can anyone explain the logic and code behind this problem.
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https://www.hackerrank.com/challenges/tutzki-and-lcs/problem
I ve been trying to solve this for hours and could'nt find any understandable online solution, Can anyone explain the logic and code behind this problem.
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You just calculate the $$$LCS$$$ as you do.
Let $$$dp1[i][j]$$$ denotes the $$$LCS$$$ of the suffix starting from $$$ith$$$ and $$$jth$$$ index of string $$$a$$$ and $$$b$$$ respectively.
And $$$dp2[i][j]$$$denotes the $$$LCS$$$ of the prefix till $$$ith$$$ and $$$jth$$$ index of string $$$a$$$ and $$$b$$$ respectively.
Now assume we are adding a character before $$$ith$$$ index of string $$$a$$$ to match it with $$$jth$$$ index of string $$$b$$$. Think of the condition for this pair of indexes to contribute to the answer.
$$$dp1[i][j+1] + dp2[i-1][j-1] == LCS$$$
If we implement that condition directly, then it will overcount. Think why is it so?
Only the different number of final $$$a$$$ string formed matters. So, there might be a possibility that you are counting the same character at an index multiple times because that character may appear at different positions in string $$$b$$$.
You can create a $$$vis[i][j]$$$ array to mark $$$1$$$ if you have placed $$$jth$$$ character at index $$$i$$$, and then count the final answer using the number of elements that are marked $$$1$$$ in the $$$vis$$$ array.
Thank you so much brother. I understood your explanation.