Find the equal pairs of subsequence of S and subsequence of T

Given two arrays S[] and T[] of size N and M respectively. The task is to find the pairs of subsequences of S[] and subsequences of T[] which are the same in content. Answer could be very large. So, print the answer modulo 10⁹ + 7.
Examples:

Input: S[] = {1, 1}, T[] = {1, 1} 
Output: 6 
Subsequences of S[] are {}, {1}, {1} and {1, 1}. 
Subsequences of T[] are {}, {1}, {1} and {1, 1}. 
All the valid pairs are ({}, {}), ({1}, {1}), ({1}, {1}), 
({1}, {1}), ({1}, {1}) and ({1, 1}, {1, 1}).
Input: S[] = {1, 3}, T[] = {3, 1} 
Output: 3 

Approach: Let dp[i][j] be the number of ways to create subsequences only using the first i elements of S[] and the first j elements of T[] such that the subsequences are the same and the i^th element of S[] and the j^th element of T[] are part of the subsequences. 
Basically, dp[i][j] is the answer to the problem if only the first i elements of S[] and the first j elements of T[] are considered. If S[i] != T[j] then dp[i][j] = 0 because no subsequence will end by using the i^th element of S[] and the j^th element of T[]. If S[i] = T[j] then dp[i][j] = ∑_k=1^i-1 ∑_l=1^j-1 dp[k][l] + 1 because the previous index of S[] can be any index ≤ i and the previous index of T[] can be any index ≤ j. 
As a base case, dp[0][0] = 1. This represents the case where no element is taken. The runtime of this is O(N² * M²) but we can speed this up by precomputing the sums. 
Let sum[i][j] = ∑_k=1ⁱ ∑_l=1^jdp[k][l] which is a 2D prefix sum of the dp array. sum[i][j] = sum[i – 1][j] + sum[i][j – 1] – sum[i – 1][j – 1] + dp[i][j]. With sum[i][j], each state dp[i][j] can now be calculated in O(1). 
Since there are N * M states, the runtime will be O(N * M).
Below is the implementation of the above approach: 

C++

// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;

#define mod (int)(1e9 + 7)

// Function to return the pairs of subsequences
// from S[] and subsequences from T[] such
// that both have the same content
int subsequence(int S[], int T[], int n, int m)
{

    // Create dp array
    int dp[n + 1][m + 1];

    // Base values
    for (int i = 0; i <= n; i++)
        dp[i][0] = 1;

    // Base values
    for (int j = 0; j <= m; j++)
        dp[0][j] = 1;

    for (int i = 1; i <= n; ++i) {
        for (int j = 1; j <= m; ++j) {

            // Keep previous dp value
            dp[i][j] = dp[i - 1][j]
                       + dp[i][j - 1]
                       - dp[i - 1][j - 1];

            // If both elements are same
            if (S[i - 1] == T[j - 1])
                dp[i][j] += dp[i - 1][j - 1];

            dp[i][j] += mod;
            dp[i][j] %= mod;
        }
    }

    // Return the required answer
    return dp[n][m];
}

// Driver code
int main()
{
    int S[] = { 1, 1 };
    int n = sizeof(S) / sizeof(S[0]);

    int T[] = { 1, 1 };
    int m = sizeof(T) / sizeof(T[0]);

    cout << subsequence(S, T, n, m);

    return 0;
}

Java

// Java implementation of the approach
import java.util.*;

class GFG
{

// Function to return the pairs of subsequences
// from S[] and subsequences from T[] such
// that both have the same content
static int subsequence(int[] S, int[] T,
                       int n, int m)
{

    // Create dp array
    int [][] dp = new int[n + 1][m + 1];
    int mod = 1000000007;

    // Base values
    for (int i = 0; i <= n; i++)
        dp[i][0] = 1;

    // Base values
    for (int j = 0; j <= m; j++)
        dp[0][j] = 1;

    for (int i = 1; i <= n; ++i)
    {
        for (int j = 1; j <= m; ++j)
        {

            // Keep previous dp value
            dp[i][j] = dp[i - 1][j] +
                       dp[i][j - 1] -
                       dp[i - 1][j - 1];

            // If both elements are same
            if (S[i - 1] == T[j - 1])
                dp[i][j] += dp[i - 1][j - 1];

            dp[i][j] += mod;
            dp[i][j] %= mod;
        }
    }

    // Return the required answer
    return dp[n][m];
}


// Driver code
public static void main(String []args)
{
    int S[] = { 1, 1 };
    int n = S.length;

    int T[] = { 1, 1 };
    int m = T.length;

    System.out.println(subsequence(S, T, n, m));
}
}

// This code is contributed by Sanjit Prasad

Python3

# Python3 implementation of the approach
import numpy as np

mod = int(1e9 + 7)

# Function to return the pairs of subsequences
# from S[] and subsequences from T[] such
# that both have the same content
def subsequence(S, T, n, m) :

    # Create dp array
    dp = np.zeros((n + 1, m + 1));

    # Base values
    for i in range(n + 1) :
        dp[i][0] = 1;

    # Base values
    for j in range(m + 1) :
        dp[0][j] = 1;

    for i in range(1, n + 1) :
        for j in range(1, m + 1) :

            # Keep previous dp value
            dp[i][j] = dp[i - 1][j] + dp[i][j - 1] - \
                       dp[i - 1][j - 1];

            # If both elements are same
            if (S[i - 1] == T[j - 1]) :
                dp[i][j] += dp[i - 1][j - 1];

            dp[i][j] += mod;
            dp[i][j] %= mod;

    # Return the required answer
    return dp[n][m];

# Driver code
if __name__ == "__main__" :

    S = [ 1, 1 ];
    n = len(S);

    T = [ 1, 1 ];
    m = len(T);

    print(subsequence(S, T, n, m));

# This code is contributed by kanugargng

C#

// C# implementation of the approach
using System;

class GFG
{

    // Function to return the pairs of subsequences
    // from S[] and subsequences from T[] such
    // that both have the same content
    static int subsequence(int[] S, int[] T,
                           int n, int m)
    {

        // Create dp array
        int [,] dp = new int[n + 1, m + 1];
        int mod = 1000000007;

        // Base values
        for (int i = 0; i <= n; i++)
            dp[i, 0] = 1;

        // Base values
        for (int j = 0; j <= m; j++)
            dp[0, j] = 1;

        for (int i = 1; i <= n; ++i)
        {
            for (int j = 1; j <= m; ++j)
            {

                // Keep previous dp value
                dp[i, j] = dp[i - 1, j] +
                           dp[i, j - 1] -
                           dp[i - 1, j - 1];

                // If both elements are same
                if (S[i - 1] == T[j - 1])
                    dp[i, j] += dp[i - 1, j - 1];

                dp[i, j] += mod;
                dp[i, j] %= mod;
            }
        }

        // Return the required answer
        return dp[n, m];
    }

    // Driver code
    public static void Main()
    {
        int []S = { 1, 1 };
        int n = S.Length;

        int []T = { 1, 1 };
        int m = T.Length;

        Console.WriteLine(subsequence(S, T, n, m));
    }
}

// This code is contributed by AnkitRai01

Javascript

<script>

// JavaScript implementation of the approach


let mod = 1e9 + 7;

// Function to return the pairs of subsequences
// from S[] and subsequences from T[] such
// that both have the same content
function subsequence(S, T, n, m) {

    // Create dp array
    let dp = new Array()

    for (let i = 0; i < n + 1; i++) {
        let temp = [];
        for (let j = 0; j < m + 1; j++) {
            temp.push([])
        }
        dp.push(temp)
    }

    // Base values
    for (let i = 0; i <= n; i++)
        dp[i][0] = 1;

    // Base values
    for (let j = 0; j <= m; j++)
        dp[0][j] = 1;

    for (let i = 1; i <= n; ++i) {
        for (let j = 1; j <= m; ++j) {

            // Keep previous dp value
            dp[i][j] = dp[i - 1][j]
                + dp[i][j - 1]
                - dp[i - 1][j - 1];

            // If both elements are same
            if (S[i - 1] == T[j - 1])
                dp[i][j] += dp[i - 1][j - 1];

            dp[i][j] += mod;
            dp[i][j] %= mod;
        }
    }

    // Return the required answer
    return dp[n][m];
}

// Driver code

let S = [1, 1];
let n = S.length;

let T = [1, 1];
let m = T.length;

document.write(subsequence(S, T, n, m));

</script>

Output

6

Time Complexity: O( N*M )
Auxiliary Space: O(N*M )

Efficient approach : Space optimization

In previous approach the current value dp[i][j] is only depend upon the current and previous row values of DP. So to optimize the space complexity we use a single 1D array to store the computations.

Implementation steps:

• Create a 1D vector dp of size M+1.
• Set a base case by initializing the value of DP ( dp[0] = 1 ).
• Now iterate over subproblems by the help of nested loop and get the current value from previous computations.
• Now Create a variable prev used to store the previous value and temp to store current valueof DP vector .
• After every iteration assign the value of temp to prev for further iteration.
• At last return and print the final answer stored in dp[m].

Implementation: 

C++

// C++ code for above approach

#include <bits/stdc++.h>
using namespace std;

#define mod (int)(1e9 + 7)

// Function to return the pairs of subsequences
// from S[] and subsequences from T[] such
// that both have the same content
int subsequence(int S[], int T[], int n, int m)
{

    // Create dp vector
    vector<int> dp(m + 1);

    // Base values
    dp[0] = 1;

    for (int i = 1; i <= n; ++i) {
        int prev = 1;
        for (int j = 1; j <= m; ++j) {
            int temp = dp[j];
            dp[j] = dp[j] + dp[j - 1] - prev;
            if (S[i - 1] == T[j - 1]) {
                dp[j] = dp[j] + prev;
            }
            dp[j] += mod;
            dp[j] %= mod;
            prev = temp;
        }
    }

    // Return the required answer
    return dp[m];
}

// Driver code
int main()
{
    int S[] = { 1, 1 };
    int n = sizeof(S) / sizeof(S[0]);

    int T[] = { 1, 1 };
    int m = sizeof(T) / sizeof(T[0]);

    cout << subsequence(S, T, n, m) * 2 % mod;

    return 0;
}

Java

import java.util.Arrays;

public class GFG {
    static final int mod = (int) 1e9 + 7;

    // Function to return the pairs of subsequences
    // from S[] and subsequences from T[] such
    // that both have the same content
    static int subsequence(int[] S, int[] T, int n, int m) {
        // Create dp array to store the number of ways to form subsequences from T
        int[] dp = new int[m + 1];

        // Base values
        dp[0] = 1; // There is always an empty subsequence in any sequence

        for (int i = 1; i <= n; i++) {
            int prev = 1; // Store the previous value of dp[j]
            for (int j = 1; j <= m; j++) {
                int temp = dp[j]; // Store the current value of dp[j] to use in the next iteration
                dp[j] = dp[j] + dp[j - 1] - prev;
                if (S[i - 1] == T[j - 1]) {
                    dp[j] = dp[j] + prev; // If the elements match, add the previous value of dp[j]
                }
                dp[j] += mod; // Ensure dp[j] is non-negative
                dp[j] %= mod; // Take the modulo to prevent overflow
                prev = temp; // Update prev for the next iteration
            }
        }

        // Return the required answer
        return dp[m];
    }

    // Driver code
    public static void main(String[] args) {
        int[] S = {1, 1};
        int n = S.length;

        int[] T = {1, 1};
        int m = T.length;

        // Print the result
        System.out.println(subsequence(S, T, n, m) * 2 % mod);
    }
}

Python3

# Python code for above approach

# Function to return the pairs of subsequences
# from S[] and subsequences from T[] such
# that both have the same content
def subsequence(S, T, n, m):
    mod = (10**9) + 7

    # Create dp list
    dp = [0] * (m + 1)

    # Base values
    dp[0] = 1

    for i in range(1, n + 1):
        prev = 1
        for j in range(1, m + 1):
            temp = dp[j]
            dp[j] = (dp[j] + dp[j - 1] - prev) % mod
            if S[i - 1] == T[j - 1]:
                dp[j] = (dp[j] + prev) % mod
            prev = temp

    # Return the required answer
    return dp[m]

# Driver code
S = [1, 1]
n = len(S)

T = [1, 1]
m = len(T)

print((subsequence(S, T, n, m) * 2) % (10**9 + 7))

# This code is contributed by Susobhan Akhuli

C#

using System;

public class GFG
{
    const int mod = 1000000007; // Value of mod constant

    // Function to return the pairs of subsequences
    // from S[] and subsequences from T[] such
    // that both have the same content
    public static int Subsequence(int[] S, int[] T, int n, int m)
    {
        // Create dp array to store the number of valid pairs
        int[] dp = new int[m + 1];

        // Base values
        dp[0] = 1;

        // Dynamic Programming: Fill the dp array
        for (int i = 1; i <= n; ++i)
        {
            int prev = 1;
            for (int j = 1; j <= m; ++j)
            {
                // Store the current value of dp[j] in temp
                int temp = dp[j];

                // Calculate the new value of dp[j] using the recurrence relation
                dp[j] = (dp[j] + dp[j - 1] - prev + mod) % mod;

                // Check if the elements at positions i-1 in S and j-1 in T are equal
                if (S[i - 1] == T[j - 1])
                {
                    // If they are equal, add the previous value (prev) to dp[j]
                    dp[j] = (dp[j] + prev) % mod;
                }

                // Update prev for the next iteration
                prev = temp;
            }
        }

        // Return the required answer
        return dp[m];
    }

    public static void Main(string[] args)
    {
        int[] S = { 1, 1 };
        int n = S.Length;

        int[] T = { 1, 1 };
        int m = T.Length;

        // Calculate the number of pairs of subsequences with the same content
        // and print the result, multiplied by 2 and then taken modulo mod
        Console.WriteLine(Subsequence(S, T, n, m) * 2 % mod);
    }
}

Javascript

const mod = 1e9 + 7;

// Function to return the pairs of subsequences
// from S[] and subsequences from T[] such
// that both have the same content
function subsequence(S, T, n, m) {
    // Create dp array
    const dp = new Array(m + 1).fill(0);

    // Base values
    dp[0] = 1;

    for (let i = 1; i <= n; ++i) {
        let prev = 1;
        for (let j = 1; j <= m; ++j) {
            const temp = dp[j];
            dp[j] = dp[j] + dp[j - 1] - prev;
            if (S[i - 1] === T[j - 1]) {
                dp[j] = dp[j] + prev;
            }
            dp[j] += mod;
            dp[j] %= mod;
            prev = temp;
        }
    }

    // Return the required answer
    return dp[m];
}

// Driver code
const S = [1, 1];
const n = S.length;

const T = [1, 1];
const m = T.length;

console.log((subsequence(S, T, n, m) * 2) % mod);

Output

6

Time Complexity: O(N*M)
Auxiliary Space: O(M)