Problem E.

Let calc(i, j) be the maximum coverage length of a special symbol i in the pattern P starting from P[j]. Then for each equation of the form A = B + C, calc(A, j) = calc(B, j) + calc(C, j + calc(B, j)), and for equation A = a word over * {a, ..., z} we count it the simple way.

To speed up, use a hash table (unordered_map) to memorize calc(i, j) values.

The solution is then calc(S, 0) == strlen(P).