solution.cpp

class Solution
{
public:
    int countWays(string &s1, string &s2)
    {
        // Mod value given in the problem
        const int MOD = 1000000007;

        int n = s1.size();
        int m = s2.size();

        // dp[i][j] = ways to form first j characters of s2
        // using first i characters of s1
        vector<vector<long long>> dp(n + 1, vector<long long>(m + 1, 0));

        // Empty string can always be formed in one way
        for (int i = 0; i <= n; i++)
            dp[i][0] = 1;

        // Fill the DP table
        for (int i = 1; i <= n; i++)
        {
            for (int j = 1; j <= m; j++)
            {

                // Skip current character of s1
                dp[i][j] = dp[i - 1][j];

                // If characters match, include current character as well
                if (s1[i - 1] == s2[j - 1])
                {
                    dp[i][j] = (dp[i][j] + dp[i - 1][j - 1]) % MOD;
                }
            }
        }

        // Final answer
        return dp[n][m];
    }
};