Hi Alexhans,
check the wikipedia page for the Longest Common Subsequent problem: https://en.wikipedia.org/wiki/Longest_common_subsequence_problem
You have to find the length of the longest subsequence common to the both of the strings, and it's different from the problem of finding common substrings: unlike substrings, subsequences are not required to occupy consecutive positions.

Subsequence definition: "In mathematics, a subsequence is a sequence that can be derived from another sequence by deleting some elements without changing the order of the remaining elements".

Consider X = XMJYAUZ and Y = MZJAWXU
The longest common subsequence between X and Y is MJAU

As you can see, in the subsequences there can be missing chars (compared to the original strings), but the important thing is that any char in the common subsequence is present in both the input strings (in the same order).

counterspell:

if (Fireball *fb=dynamic_cast<Fireball*>(spell)) {
    fb->revealFirepower();
} 
else if (Frostbite *fz=dynamic_cast<Frostbite*>(spell)) {
    fz->revealFrostpower();
} 
else if (Thunderstorm *ts=dynamic_cast<Thunderstorm*>(spell)) {
    ts->revealThunderpower();
} 
else if (Waterbolt *wb=dynamic_cast<Waterbolt*>(spell)) {
    wb->revealWaterpower();
}
else { // generic spell
    string spellN = spell->revealScrollName();
    string spellJ = SpellJournal::read();
    int m=spellN.length();
    int n=spellJ.length();
    int array[m+1][n+1]; 

    // solve for LCS
    for (int i=0;i<=m;i++) array[i][0]=0;
    for (int j=0;j<=n;j++) array[0][j]=0;
    for (int i=1;i<=m;i++) {
        for (int j=1;j<=n;j++) {
            if (spellN[i-1] == spellJ[j-1])
                array[i][j]=array[i-1][j-1]+1;
            else
                array[i][j]=max(array[i][j-1],array[i-1][j]);
        }
    }  
    cout << array[m][n] << endl; 
}