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Find length of longest subsequence of one string which is substring of another string
  • Difficulty Level : Medium
  • Last Updated : 06 May, 2021

Given two string X and Y. The task is to find the length of longest subsequence of string X which is substring in sequence Y.
Examples: 
 

Input : X = "ABCD",  Y = "BACDBDCD"
Output : 3
"ACD" is longest subsequence of X which
is substring of Y.

Input : X = "A",  Y = "A"
Output : 1

 

Method 1 (Brute Force): 
Use brute force to find all the subsequence of X and for each subsequence check whether it is substring of Y or not. If it is substring of Y, maintain a maximum length variable and compare length with it.
Method 2: (Dynamic Programming): 
Let n be length of X and m be length of Y. Create a 2D array ‘dp[][]’ of m + 1 rows and n + 1 columns. Value dp[i][j] is maximum length of subsequence of X[0….j] which is substring of Y[0….i]. Now for each cell of dp[][] fill value as : 
 

for (i = 1 to m)
  for (j = 1 to n)
    if (x[i-1] == y[j - 1])
      dp[i][j] = dp[i-1][j-1] + 1;
    else
      dp[i][j] = dp[i][j-1];

And finally, the length of the longest subsequence of x which is substring of y is max(dp[i][n]) where 1 <= i <= m.
Below is implementation this approach:
 



C/C++


// C++ program to find maximum length of
// subsequence of a string X such it is
// substring in another string Y.
#include <bits/stdc++.h>
#define MAX 1000
using namespace std;

// Return the maximum size of substring of
// X which is substring in Y.
int maxSubsequenceSubstring(char x[], char y[],
                            int n, int m)
{
    int dp[MAX][MAX];

    // Initialize the dp[][] to 0.
    for (int i = 0; i <= m; i++)
        for (int j = 0; j <= n; j++)
            dp[i][j] = 0;

    // Calculating value for each element.
    for (int i = 1; i <= m; i++) {
        for (int j = 1; j <= n; j++) {

            // If alphabet of string X and Y are
            // equal make dp[i][j] = 1 + dp[i-1][j-1]
            if (x[j - 1] == y[i - 1])
                dp[i][j] = 1 + dp[i - 1][j - 1];

            // Else copy the previous value in the
            // row i.e dp[i-1][j-1]
            else
                dp[i][j] = dp[i][j - 1];
        }
    }

    // Finding the maximum length.
    int ans = 0;
    for (int i = 1; i <= m; i++)
        ans = max(ans, dp[i][n]);

    return ans;
}

// Driver Program
int main()
{
    char x[] = "ABCD";
    char y[] = "BACDBDCD";
    int n = strlen(x), m = strlen(y);
    cout << maxSubsequenceSubstring(x, y, n, m);
    return 0;
}

Java




// Java program to find maximum length of
// subsequence of a string X such it is
// substring in another string Y.
 
public class GFG
{
    static final int MAX = 1000;
     
    // Return the maximum size of substring of
    // X which is substring in Y.
    static int maxSubsequenceSubstring(char x[], char y[],
                                int n, int m)
    {
        int dp[][] = new int[MAX][MAX];
      
        // Initialize the dp[][] to 0.
        for (int i = 0; i <= m; i++)
            for (int j = 0; j <= n; j++)
                dp[i][j] = 0;
      
        // Calculating value for each element.
        for (int i = 1; i <= m; i++) {
            for (int j = 1; j <= n; j++) {
      
                // If alphabet of string X and Y are
                // equal make dp[i][j] = 1 + dp[i-1][j-1]
                if (x[j - 1] == y[i - 1])
                    dp[i][j] = 1 + dp[i - 1][j - 1];
      
                // Else copy the previous value in the
                // row i.e dp[i-1][j-1]
                else
                    dp[i][j] = dp[i][j - 1];
            }
        }
      
        // Finding the maximum length.
        int ans = 0;
        for (int i = 1; i <= m; i++)
            ans = Math.max(ans, dp[i][n]);
      
        return ans;
    }
     
    // Driver Method
    public static void main(String[] args)
    {
        char x[] = "ABCD".toCharArray();
        char y[] = "BACDBDCD".toCharArray();
        int n = x.length, m = y.length;
        System.out.println(maxSubsequenceSubstring(x, y, n, m));
    }
}

Python3




# Python3 program to find maximum
# length of subsequence of a string
# X such it is substring in another
# string Y.
 
MAX = 1000
 
# Return the maximum size of
# substring of X which is
# substring in Y.
def maxSubsequenceSubstring(x, y, n, m):
    dp = [[0 for i in range(MAX)]
             for i in range(MAX)]
              
    # Initialize the dp[][] to 0.
 
    # Calculating value for each element.
    for i in range(1, m + 1):
        for j in range(1, n + 1):
             
            # If alphabet of string
            # X and Y are equal make
            # dp[i][j] = 1 + dp[i-1][j-1]
            if(x[j - 1] == y[i - 1]):
                dp[i][j] = 1 + dp[i - 1][j - 1]
 
            # Else copy the previous value
            # in the row i.e dp[i-1][j-1]
            else:
                dp[i][j] = dp[i][j - 1]
                 
    # Finding the maximum length
    ans = 0
    for i in range(1, m + 1):
        ans = max(ans, dp[i][n])
    return ans
 
# Driver Code
x = "ABCD"
y = "BACDBDCD"
n = len(x)
m = len(y)
print(maxSubsequenceSubstring(x, y, n, m))
 
# This code is contributed
# by sahilshelangia

C#




// C# program to find maximum length of
// subsequence of a string X such it is
// substring in another string Y.
using System;
 
public class GFG
{
    static int MAX = 1000;
     
    // Return the maximum size of substring of
    // X which is substring in Y.
    static int maxSubsequenceSubstring(string x, string y,
                                            int n, int m)
    {
        int[ ,]dp = new int[MAX, MAX];
     
        // Initialize the dp[][] to 0.
        for (int i = 0; i <= m; i++)
            for (int j = 0; j <= n; j++)
                dp[i, j] = 0;
     
        // Calculating value for each element.
        for (int i = 1; i <= m; i++) {
            for (int j = 1; j <= n; j++) {
     
                // If alphabet of string X and Y are
                // equal make dp[i][j] = 1 + dp[i-1][j-1]
                if (x[j - 1] == y[i - 1])
                    dp[i, j] = 1 + dp[i - 1, j - 1];
     
                // Else copy the previous value in the
                // row i.e dp[i-1][j-1]
                else
                    dp[i, j] = dp[i, j - 1];
            }
        }
     
        // Finding the maximum length.
        int ans = 0;
         
        for (int i = 1; i <= m; i++)
            ans = Math.Max(ans, dp[i,n]);
     
        return ans;
    }
     
    // Driver Method
    public static void Main()
    {
        string x = "ABCD";
        string y = "BACDBDCD";
        int n = x.Length, m = y.Length;
         
        Console.WriteLine(maxSubsequenceSubstring(x,
                                            y, n, m));
    }
}
 
// This code is contributed by vt_m.

PHP




<?php
// PHP program to find maximum length of
// subsequence of a string X such it is
// substring in another string Y.
 
// Return the maximum size of substring of
// X which is substring in Y.
function maxSubsequenceSubstring($x, $y,
                                 $n, $m)
{
    $dp;
 
    // Initialize the dp[][] to 0.
    for ($i = 0; $i <= $m; $i++)
        for ($j = 0; $j <= $n; $j++)
            $dp[$i][$j] = 0;
 
    // Calculating value for each element.
    for ($i = 1; $i <= $m; $i++) {
        for ( $j = 1; $j <= $n; $j++) {
 
            // If alphabet of string
            // X and Y are equal make
            // dp[i][j] = 1 + dp[i-1][j-1]
            if ($x[$j - 1] == $y[$i - 1])
                $dp[$i][$j] = 1 + $dp[$i - 1][$j - 1];
 
            // Else copy the previous
            // value in the
            // row i.e dp[i-1][j-1]
            else
                $dp[$i][$j] = $dp[$i][$j - 1];
        }
    }
 
    // Finding the maximum length.
    $ans = 0;
    for ( $i = 1; $i <= $m; $i++)
        $ans = max($ans, $dp[$i][$n]);
 
    return $ans;
}
 
// Driver Code
{
    $x = "ABCD";
    $y = "BACDBDCD";
    $n = strlen($x); $m = strlen($y);
    echo maxSubsequenceSubstring($x, $y, $n, $m);
    return 0;
}
 
// This code is contributed by nitin mittal
?>

Javascript




<script>
 
 
// Javascript program to find maximum length of
// subsequence of a string X such it is
// substring in another string Y.
var MAX = 1000;
 
// Return the maximum size of substring of
// X which is substring in Y.
function maxSubsequenceSubstring(x, y, n, m)
{
    var dp = Array.from(Array(MAX), ()=> Array(MAX));
 
    // Initialize the dp[][] to 0.
    for (var i = 0; i <= m; i++)
        for (var j = 0; j <= n; j++)
            dp[i][j] = 0;
 
    // Calculating value for each element.
    for (var i = 1; i <= m; i++) {
        for (var j = 1; j <= n; j++) {
 
            // If alphabet of string X and Y are
            // equal make dp[i][j] = 1 + dp[i-1][j-1]
            if (x[j - 1] == y[i - 1])
                dp[i][j] = 1 + dp[i - 1][j - 1];
 
            // Else copy the previous value in the
            // row i.e dp[i-1][j-1]
            else
                dp[i][j] = dp[i][j - 1];
        }
    }
 
    // Finding the maximum length.
    var ans = 0;
    for (var i = 1; i <= m; i++)
        ans = Math.max(ans, dp[i][n]);
 
    return ans;
}
 
// Driver Program
var x = "ABCD";
var y = "BACDBDCD";
var n = x.length, m = y.length;
document.write( maxSubsequenceSubstring(x, y, n, m));
 
</script>

Output: 
 

3

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