Leetcode 2930. Number of Strings Which Can Be Rearranged to Contain Substring

Count the number of length-n lowercase strings whose character multiset contains at least two 'e's, one 'l' and one 't' (so they can be rearranged to contain "leet"), computed modulo 1e9+7; this reduces to a combinatorics/inclusion–exclusion problem on letter-frequency counts for n up to 1e5.