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# LCS (Longest Common Subsequence) of three strings

• Difficulty Level : Medium
• Last Updated : 17 Jun, 2021

Given 3 strings of all having length < 100,the task is to find the longest common sub-sequence in all three given sequences.
Examples:

Input : str1 = "geeks"
str2 = "geeksfor"
str3 = "geeksforgeeks"
Output : 5
Longest common subsequence is "geeks"
i.e., length = 5

Input : str1 = "abcd1e2"
str2 = "bc12ea"
str3 = "bd1ea"
Output : 3
Longest common subsequence is "b1e"
i.e. length = 3.

This problem is simply an extension of LCS
Let the input sequences be X[0..m-1], Y[0..n-1] and Z[0..o-1] of lengths m, n and o respectively. And let L(X[0..m-1], Y[0..n-1], Z[0..o-1]) be the lengths of LCS of the three sequences X, Y and Z. Following is the implementation:

The idea is to take a 3D array to store the
length of common subsequence in all 3 given
sequences i. e., L[m + 1][n + 1][o + 1]

1- If any of the string is empty then there
is no common subsequence at all then
L[i][j][k] = 0

2- If the characters of all sequences match
(or X[i] == Y[j] ==Z[k]) then
L[i][j][k] = 1 + L[i-1][j-1][k-1]

3- If the characters of both sequences do
not match (or X[i] != Y[j] || X[i] != Z[k]
|| Y[j] !=Z[k]) then
L[i][j][k] = max(L[i-1][j][k],
L[i][j-1][k],
L[i][j][k-1])

Below is implementation of above idea.

## C++

 // C++ program to find LCS of three strings#includeusing namespace std; /* Returns length of LCS for X[0..m-1], Y[0..n-1]   and Z[0..o-1] */int lcsOf3( string X, string Y, string Z, int m,                               int n, int o){    int L[m+1][n+1][o+1];     /* Following steps build L[m+1][n+1][o+1] in       bottom up fashion. Note that L[i][j][k]       contains length of LCS of X[0..i-1] and       Y[0..j-1]  and Z[0.....k-1]*/    for (int i=0; i<=m; i++)    {        for (int j=0; j<=n; j++)        {            for (int k=0; k<=o; k++)            {                if (i == 0 || j == 0||k==0)                    L[i][j][k] = 0;                 else if (X[i-1] == Y[j-1] && X[i-1]==Z[k-1])                    L[i][j][k] = L[i-1][j-1][k-1] + 1;                 else                    L[i][j][k] = max(max(L[i-1][j][k],                                         L[i][j-1][k]),                                     L[i][j][k-1]);            }        }    }     /* L[m][n][o] contains length of LCS for      X[0..n-1] and Y[0..m-1] and Z[0..o-1]*/    return L[m][n][o];} /* Driver program to test above function */int main(){    string X = "AGGT12";    string Y = "12TXAYB";    string Z = "12XBA";     int m = X.length();    int n = Y.length();    int o = Z.length();     cout << "Length of LCS is " << lcsOf3(X, Y,                                    Z, m, n, o);     return 0;}

## Java

 // Java program to find LCS of three stringspublic class LCS_3Strings {          /* Returns length of LCS for X[0..m-1], Y[0..n-1]       and Z[0..o-1] */    static int lcsOf3(String X, String Y, String Z, int m,                                   int n, int o)    {        int[][][] L = new int[m+1][n+1][o+1];              /* Following steps build L[m+1][n+1][o+1] in           bottom up fashion. Note that L[i][j][k]           contains length of LCS of X[0..i-1] and           Y[0..j-1]  and Z[0.....k-1]*/        for (int i=0; i<=m; i++)        {            for (int j=0; j<=n; j++)            {                for (int k=0; k<=o; k++)                {                    if (i == 0 || j == 0||k==0)                        L[i][j][k] = 0;                          else if (X.charAt(i - 1) == Y.charAt(j - 1)                                && X.charAt(i - 1)==Z.charAt(k - 1))                        L[i][j][k] = L[i-1][j-1][k-1] + 1;                          else                        L[i][j][k] = Math.max(Math.max(L[i-1][j][k],                                             L[i][j-1][k]),                                         L[i][j][k-1]);                }            }        }              /* L[m][n][o] contains length of LCS for          X[0..n-1] and Y[0..m-1] and Z[0..o-1]*/        return L[m][n][o];    }          /* Driver program to test above function */    public static void main(String args[])    {        String X = "AGGT12";        String Y = "12TXAYB";        String Z = "12XBA";              int m = X.length();        int n = Y.length();        int o = Z.length();              System.out.println("Length of LCS is " +                                lcsOf3(X, Y,Z, m, n, o));          }}// This code is contributed by Sumit Ghosh

## Python3

 # Python program to find# LCS of three strings # Returns length of LCS# for X[0..m-1], Y[0..n-1]# and Z[0..o-1]def lcsOf3(X, Y, Z, m, n, o):         L = [[[0 for i in range(o+1)] for j in range(n+1)]         for k in range(m+1)]     ''' Following steps build L[m+1][n+1][o+1] in    bottom up fashion. Note that L[i][j][k]    contains length of LCS of X[0..i-1] and    Y[0..j-1] and Z[0.....k-1] '''    for i in range(m+1):        for j in range(n+1):            for k in range(o+1):                if (i == 0 or j == 0 or k == 0):                    L[i][j][k] = 0                                     elif (X[i-1] == Y[j-1] and                      X[i-1] == Z[k-1]):                    L[i][j][k] = L[i-1][j-1][k-1] + 1                 else:                    L[i][j][k] = max(max(L[i-1][j][k],                    L[i][j-1][k]),                                    L[i][j][k-1])     # L[m][n][o] contains length of LCS for    # X[0..n-1] and Y[0..m-1] and Z[0..o-1]    return L[m][n][o] # Driver program to test above function X = 'AGGT12'Y = '12TXAYB'Z = '12XBA' m = len(X)n = len(Y)o = len(Z) print('Length of LCS is', lcsOf3(X, Y, Z, m, n, o)) # This code is contributed by Soumen Ghosh.

## C#

 // C# program to find// LCS of three stringsusing System; class GFG{         /* Returns length of LCS    for X[0..m-1], Y[0..n-1]    and Z[0..o-1] */    static int lcsOf3(String X, String Y,                      String Z, int m,                      int n, int o)    {        int [,,]L = new int[m + 1,                            n + 1, o + 1];             /* Following steps build        L[m+1][n+1][o+1] in bottom        up fashion. Note that        L[i][j][k] contains length        of LCS of X[0..i-1] and        Y[0..j-1] and Z[0.....k-1]*/        for (int i = 0; i <= m; i++)        {            for (int j = 0; j <= n; j++)            {                for (int k = 0; k <= o; k++)                {                    if (i == 0 ||                        j == 0 || k == 0)                        L[i, j, k] = 0;                         else if (X[i - 1] == Y[j - 1] &&                             X[i - 1] == Z[k - 1])                        L[i, j, k] = L[i - 1,                                       j - 1,                                       k - 1] + 1;                         else                        L[i, j, k] = Math.Max(Math.Max(L[i - 1, j, k],                                                       L[i, j - 1, k]),                                                       L[i, j, k - 1]);                }            }        }             /* L[m][n][o] contains length        of LCS for X[0..n-1] and        Y[0..m-1] and Z[0..o-1]*/        return L[m, n, o];    }         // Driver Code    public static void Main()    {        string X = "AGGT12";        string Y = "12TXAYB";        string Z = "12XBA";             int m = X.Length;        int n = Y.Length;        int o = Z.Length;             Console.Write("Length of LCS is " +                       lcsOf3(X, Y, Z, m, n, o));    }} // This code is contributed// by shiv_bhakt.



## Javascript



Output:

Length of LCS is 2

Another approach: (Using recursion)

## C++

 // C++ program to find LCS of three strings#includeusing namespace std;     string X = "AGGT12";    string Y = "12TXAYB";    string Z = "12XBA"; int dp[100][100][100]; /* Returns length of LCS for X[0..m-1], Y[0..n-1]and Z[0..o-1] */int lcsOf3(int i, int j,int k){    if(i==-1||j==-1||k==-1)        return 0;    if(dp[i][j][k]!=-1)        return dp[i][j][k];         if(X[i]==Y[j] && Y[j]==Z[k])        return dp[i][j][k] = 1+lcsOf3(i-1,j-1,k-1);    else        return dp[i][j][k] = max(max(lcsOf3(i-1,j,k),                            lcsOf3(i,j-1,k)),lcsOf3(i,j,k-1));} // Driver codeint main(){    memset(dp, -1,sizeof(dp));    int m = X.length();    int n = Y.length();    int o = Z.length();     cout << "Length of LCS is " << lcsOf3(m-1,n-1,o-1);// this code is contributed by Kushdeep Mittal}

## Java

 // Java program to find LCS of three stringsclass GFG{     static String X = "AGGT12";    static String Y = "12TXAYB";    static String Z = "12XBA";     static int[][][] dp = new int[100][100][100];     /* Returns length of LCS for X[0..m-1],    Y[0..n-1] and Z[0..o-1] */    static int lcsOf3(int i, int j, int k)    {        if (i == -1 || j == -1 || k == -1)        {            return 0;        }        if (dp[i][j][k] != -1)        {            return dp[i][j][k];        }         if (X.charAt(i) == Y.charAt(j) &&            Y.charAt(j) == Z.charAt(k))        {            return dp[i][j][k] = 1 + lcsOf3(i - 1, j - 1, k - 1);        } else {            return dp[i][j][k] = Math.max(Math.max(lcsOf3(i - 1, j, k),                                lcsOf3(i, j - 1, k)),                                lcsOf3(i, j, k - 1));        }    }     // Driver code    public static void main(String[] args)    {         for (int i = 0; i < 100; i++)        {            for (int j = 0; j < 100; j++)            {                for (int k = 0; k < 100; k++)                {                    dp[i][j][k] = -1;                }            }        }        int m = X.length();        int n = Y.length();        int o = Z.length();         System.out.print("Length of LCS is "                + lcsOf3(m - 1, n - 1, o - 1));    }} // This code has been contributed by 29AjayKumar

## Python3

 # Python3 program to find LCS of# three stringsX = "AGGT12"Y = "12TXAYB"Z = "12XBA" dp = [[[-1 for i in range(100)]           for j in range(100)]           for k in range(100)]         # Returns length of LCS for# X[0..m-1], Y[0..n-1] and Z[0..o-1]def lcsOf3(i, j, k) :     if(i == -1 or j == -1 or k == -1) :        return 0             if(dp[i][j][k] != -1) :        return dp[i][j][k]         if(X[i] == Y[j] and Y[j] == Z[k]) :        dp[i][j][k] = 1 + lcsOf3(i - 1,                                 j - 1, k - 1)        return dp[i][j][k]             else :        dp[i][j][k] = max(max(lcsOf3(i - 1, j, k),                              lcsOf3(i, j - 1, k)),                              lcsOf3(i, j, k - 1))                 return dp[i][j][k] # Driver codeif __name__ == "__main__" :    m = len(X)    n = len(Y)    o = len(Z)     print("Length of LCS is",           lcsOf3(m - 1, n - 1, o - 1))     # This code is contributed by Ryuga

## C#

 // C# program to find LCS of three stringsusing System; class GFG{static string X = "AGGT12";static string Y = "12TXAYB";static string Z = "12XBA"; static int[,,] dp = new int[100, 100, 100]; /* Returns length of LCS for X[0..m-1],Y[0..n-1] and Z[0..o-1] */static int lcsOf3(int i, int j, int k){    if(i == -1 || j == -1 || k == -1)        return 0;    if(dp[i, j, k] != -1)        return dp[i, j, k];         if(X[i] == Y[j] && Y[j] == Z[k])        return dp[i, j, k] = 1 + lcsOf3(i - 1, j - 1, k - 1);    else        return dp[i, j, k] = Math.Max(Math.Max(lcsOf3(i - 1, j, k),                                               lcsOf3(i, j - 1, k)),                                               lcsOf3(i, j, k - 1));} // Driver codestatic void Main(){    for(int i = 0; i < 100; i++)        for(int j = 0; j < 100; j++)            for(int k = 0; k < 100; k++)                dp[i, j, k] = -1;    int m = X.Length;    int n = Y.Length;    int o = Z.Length;     Console.Write("Length of LCS is " +                   lcsOf3(m - 1, n - 1, o - 1));}} // This code is contributed by DrRoot_



## Javascript



Output:

Length of LCS is 2

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