What is longest common subsequence?
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Ethan Martinez
Works at the International Committee of the Red Cross, Lives in Geneva, Switzerland.
Hello, I'm a computational expert with a focus on algorithms and data structures. Let's delve into the concept of the Longest Common Subsequence (LCS).
The Longest Common Subsequence problem is a classic problem in computer science that involves finding the longest subsequence common to all sequences in a set of sequences (typically two). A subsequence is a sequence that can be derived from another sequence by deleting some or no elements without changing the order of the remaining elements. The LCS problem is widely used in various fields, such as bioinformatics for sequence alignment, in version control systems for file comparison, and in the study of string processing algorithms.
To understand LCS, let's consider two sequences:
\[ X = \{a, b, c, d, e\} \]
\[ Y = \{a, c, e\} \]
The common subsequences of X and Y are many, including `{}`, `{a}`, `{a, c}`, `{a, e}`, `{a, c, e}`, `{c}`, `{c, e}`, and `{e}`. Among these, `{a, c, e}` is the longest, and thus it is the LCS of the two sequences.
The problem of finding the LCS can be solved using dynamic programming, which is a method for solving complex problems by breaking them down into simpler subproblems and storing the results of these subproblems to avoid redundant computations.
Here's a step-by-step approach to finding the LCS using dynamic programming:
1. Initialization: Create a 2D array `dp` where `dp[i][j]` will store the length of the LCS of the sequences `X[0..i-1]` and `Y[0..j-1]`.
2. Base Case: If either `i` or `j` is 0, then `dp[i][j]` should be 0 because a sequence of length 0 has no common subsequence with any other sequence.
3. Filling the Table: For each `i` and `j` where `1 <= i <= m` and `1 <= j <= n` (assuming `m` and `n` are the lengths of sequences X and Y respectively), do the following:
- If `X[i-1] == Y[j-1]`, then `dp[i][j] = dp[i-1][j-1] + 1` because the current elements of X and Y are part of the LCS.
- If `X[i-1] != Y[j-1]`, then `dp[i][j] = max(dp[i-1][j], dp[i][j-1])` because we need to consider the LCS without the current element from either X or Y.
4. Traceback: To find the LCS itself and not just its length, you can start from `dp[m][n]` and trace back through the table based on the following rules:
- If `X[i-1] == Y[j-1]`, this element is part of the LCS, so move diagonally to `dp[i-1][j-1]`.
- If `X[i-1] != Y[j-1]`, move in the direction that gives a higher value, either to `dp[i-1][j]` or `dp[i][j-1]`.
5. Result: The LCS can be read by tracing back from `dp[m][n]` to `dp[0][0]`.
The time complexity of this dynamic programming approach is O(m * n), where `m` and `n` are the lengths of the two sequences. The space complexity is also O(m * n) due to the storage requirements of the 2D array.
Now, let's move on to the translation of this explanation into Chinese.
The Longest Common Subsequence problem is a classic problem in computer science that involves finding the longest subsequence common to all sequences in a set of sequences (typically two). A subsequence is a sequence that can be derived from another sequence by deleting some or no elements without changing the order of the remaining elements. The LCS problem is widely used in various fields, such as bioinformatics for sequence alignment, in version control systems for file comparison, and in the study of string processing algorithms.
To understand LCS, let's consider two sequences:
\[ X = \{a, b, c, d, e\} \]
\[ Y = \{a, c, e\} \]
The common subsequences of X and Y are many, including `{}`, `{a}`, `{a, c}`, `{a, e}`, `{a, c, e}`, `{c}`, `{c, e}`, and `{e}`. Among these, `{a, c, e}` is the longest, and thus it is the LCS of the two sequences.
The problem of finding the LCS can be solved using dynamic programming, which is a method for solving complex problems by breaking them down into simpler subproblems and storing the results of these subproblems to avoid redundant computations.
Here's a step-by-step approach to finding the LCS using dynamic programming:
1. Initialization: Create a 2D array `dp` where `dp[i][j]` will store the length of the LCS of the sequences `X[0..i-1]` and `Y[0..j-1]`.
2. Base Case: If either `i` or `j` is 0, then `dp[i][j]` should be 0 because a sequence of length 0 has no common subsequence with any other sequence.
3. Filling the Table: For each `i` and `j` where `1 <= i <= m` and `1 <= j <= n` (assuming `m` and `n` are the lengths of sequences X and Y respectively), do the following:
- If `X[i-1] == Y[j-1]`, then `dp[i][j] = dp[i-1][j-1] + 1` because the current elements of X and Y are part of the LCS.
- If `X[i-1] != Y[j-1]`, then `dp[i][j] = max(dp[i-1][j], dp[i][j-1])` because we need to consider the LCS without the current element from either X or Y.
4. Traceback: To find the LCS itself and not just its length, you can start from `dp[m][n]` and trace back through the table based on the following rules:
- If `X[i-1] == Y[j-1]`, this element is part of the LCS, so move diagonally to `dp[i-1][j-1]`.
- If `X[i-1] != Y[j-1]`, move in the direction that gives a higher value, either to `dp[i-1][j]` or `dp[i][j-1]`.
5. Result: The LCS can be read by tracing back from `dp[m][n]` to `dp[0][0]`.
The time complexity of this dynamic programming approach is O(m * n), where `m` and `n` are the lengths of the two sequences. The space complexity is also O(m * n) due to the storage requirements of the 2D array.
Now, let's move on to the translation of this explanation into Chinese.
2024-05-22 17:30:38
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Works at the International Criminal Court, Lives in The Hague, Netherlands.
Longest common subsequence (LCS) of 2 sequences is a subsequence, with maximal length, which is common to both the sequences. Given two sequences of integers, and , find the longest common subsequence and print it as a line of space-separated integers.
2023-06-15 13:23:27
Mia Anderson
QuesHub.com delivers expert answers and knowledge to you.
Longest common subsequence (LCS) of 2 sequences is a subsequence, with maximal length, which is common to both the sequences. Given two sequences of integers, and , find the longest common subsequence and print it as a line of space-separated integers.
