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# Python Program for Longest Common Subsequence

• Difficulty Level : Medium
• Last Updated : 18 Apr, 2020

LCS Problem Statement: Given two sequences, find the length of longest subsequence present in both of them. A subsequence is a sequence that appears in the same relative order, but not necessarily contiguous. For example, “abc”, “abg”, “bdf”, “aeg”, ‘”acefg”, .. etc are subsequences of “abcdefg”. So a string of length n has 2^n different possible subsequences.

It is a classic computer science problem, the basis of diff (a file comparison program that outputs the differences between two files), and has applications in bioinformatics.

Examples:
LCS for input Sequences “ABCDGH” and “AEDFHR” is “ADH” of length 3.
LCS for input Sequences “AGGTAB” and “GXTXAYB” is “GTAB” of length 4.

Let the input sequences be X[0..m-1] and Y[0..n-1] of lengths m and n respectively. And let L(X[0..m-1], Y[0..n-1]) be the length of LCS of the two sequences X and Y. Following is the recursive definition of L(X[0..m-1], Y[0..n-1]).

If last characters of both sequences match (or X[m-1] == Y[n-1]) then
L(X[0..m-1], Y[0..n-1]) = 1 + L(X[0..m-2], Y[0..n-2])

If last characters of both sequences do not match (or X[m-1] != Y[n-1]) then
L(X[0..m-1], Y[0..n-1]) = MAX ( L(X[0..m-2], Y[0..n-1]), L(X[0..m-1], Y[0..n-2])

 `# A Naive recursive Python implementation of LCS problem`` ` `def` `lcs(X, Y, m, n):`` ` `    ``if` `m ``=``=` `0` `or` `n ``=``=` `0``:``       ``return` `0``;``    ``elif` `X[m``-``1``] ``=``=` `Y[n``-``1``]:``       ``return` `1` `+` `lcs(X, Y, m``-``1``, n``-``1``);``    ``else``:``       ``return` `max``(lcs(X, Y, m, n``-``1``), lcs(X, Y, m``-``1``, n));`` ` ` ` `# Driver program to test the above function``X ``=` `"AGGTAB"``Y ``=` `"GXTXAYB"``print` `(``"Length of LCS is "``, lcs(X, Y, ``len``(X), ``len``(Y)))`
Output:
```Length of LCS is  4
```

Following is a tabulated implementation for the LCS problem.

 `# Dynamic Programming implementation of LCS problem`` ` `def` `lcs(X, Y):``    ``# find the length of the strings``    ``m ``=` `len``(X)``    ``n ``=` `len``(Y)`` ` `    ``# declaring the array for storing the dp values``    ``L ``=` `[[``None``]``*``(n ``+` `1``) ``for` `i ``in` `range``(m ``+` `1``)]`` ` `    ``"""Following steps build L[m + 1][n + 1] in bottom up fashion``    ``Note: L[i][j] contains length of LCS of X[0..i-1]``    ``and Y[0..j-1]"""``    ``for` `i ``in` `range``(m ``+` `1``):``        ``for` `j ``in` `range``(n ``+` `1``):``            ``if` `i ``=``=` `0` `or` `j ``=``=` `0` `:``                ``L[i][j] ``=` `0``            ``elif` `X[i``-``1``] ``=``=` `Y[j``-``1``]:``                ``L[i][j] ``=` `L[i``-``1``][j``-``1``]``+``1``            ``else``:``                ``L[i][j] ``=` `max``(L[i``-``1``][j], L[i][j``-``1``])`` ` `    ``# L[m][n] contains the length of LCS of X[0..n-1] & Y[0..m-1]``    ``return` `L[m][n]``# end of function lcs`` ` ` ` `# Driver program to test the above function``X ``=` `"AGGTAB"``Y ``=` `"GXTXAYB"``print``(``"Length of LCS is "``, lcs(X, Y))`` ` `# This code is contributed by Nikhil Kumar Singh(nickzuck_007)`
Output:
```Length of LCS is  4
```

Please refer complete article on Dynamic Programming | Set 4 (Longest Common Subsequence) for more details!

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