Doubt in Required Substring CSES problem

#	User	Rating
1	tourist	4009
2	jiangly	3831
3	Radewoosh	3646
4	jqdai0815	3620
4	Benq	3620
6	orzdevinwang	3529
7	ecnerwala	3446
8	Um_nik	3396
9	gamegame	3386
10	ksun48	3373

#	User	Contrib.
1	cry	164
1	maomao90	164
3	Um_nik	163
4	atcoder_official	160
5	-is-this-fft-	158
6	awoo	157
7	adamant	156
8	TheScrasse	154
8	nor	154
10	Dominater069	153

Hello everyone, I am trying to solve Required Substring problem from CSES.

First of all, I tried to solve it using combinatorics. My idea was that consider $$$n=6$$$ and string $$$s=$$$ABCDB with 5 characters, then the number of ways we can make $$$n$$$ character string with string $$$s$$$ in it, would be to fix string $$$s$$$ at first position in $$$n$$$ characters and fill rest positions with $$$26$$$ characters, giving $$$numberOfWays = 26$$$. Then, we can shift the substring for $$$(n - m)$$$, i.e, $$$6 - 5 = 1$$$ times to right, and we would get $$$numberOfWays = (n - m + 1) * 26^{(n - m)}$$$, which would, for sample test case, result in $$$numberOfWays = (6 - 5 + 1) * 26^{(6 - 5)} = 52$$$

But, it failed for this test case $$$n=6$$$ and $$$s=$$$ AA. And, I am not able to figure it out, why this is not working because of the large output.

Then, I peeked the solution here, and tried it to code myself, but in the end it is not working. My idea was same as the solution, that is, to build string character by character, and we track progress of building required string as our substring.

My Code

int recurse(int in, int maximumMatchingPrefixLength, int n, int m, int numberOfRequiredSubString) {
    if(in == n) {
        return numberOfRequiredSubString > 0;
    }

    int ans = 0;

    // matching case
    if(maximumMatchingPrefixLength+1 >= m) {
        // insert the last character, and now maximum matching prefix would become 0
        // and number of required substring would increase by 1
        ans += recurse(in+1, 0, n, m, numberOfRequiredSubString+1);
    }
    else {
        // insert that character, which is matching to next character after maximum matching prefix
        // for ex, required substring = ABC
        // and current string is XGAB
        // then, we are inserting C here
        ans += recurse(in+1, maximumMatchingPrefixLength+1, n, m, numberOfRequiredSubString);
    }

    // non matching case
    // insert all those characters, which are coming after any prefix length of Required string
    // and recurse for next position, with that prefix length
    set<char> uc;
    uc.insert(s[maximumMatchingPrefixLength]);
    maximumMatchingPrefixLength = lps[maximumMatchingPrefixLength - 1];
    while(maximumMatchingPrefixLength > 0) {
        uc.insert(s[maximumMatchingPrefixLength]);
        ans += recurse(in+1, lps[maximumMatchingPrefixLength-1], n, m, numberOfRequiredSubString);
        maximumMatchingPrefixLength = lps[maximumMatchingPrefixLength-1];
    }    
    
    // insert other characters, which did not come till now
    ans += (26 - uc.size()) * recurse(in+1, 0, n, m, numberOfRequiredSubString);

    return ans;

}

And, the Solution is following slightly different approach.

Solution

ll solve(int i, int n, int j, string &s)
{
    if(done[i][j]) return dp[i][j];
    done[i][j] = true;
    if(i == n) return dp[i][j] = (j==(int)s.size()?1:0);
    if(j == (int)s.size())
    {
        return dp[i][j] = (mxK*solve(i+1,n,j,s))%mod;
    }
    ll ans = 0;
    int t;
    for(int k = 0; k < mxK; k++)
    {
        t = j;
        while(true)
        {
            if(k == s[t]-'A')
            {
                t++;
                break;
            }
            else if(t)
            {
                t = kmp[t-1];
            }
            else break;
        }
        ans += solve(i+1,n,t,s);
        ans %= mod;
    }
    return dp[i][j] = ans;
}

I think my code is faulty at many places but if someone can tell what mistakes my code is doing, or even the main problem of my code, then it would be really helpful for me.

Thanks

History