You are given two strings word1 and word2.
A string x is called valid if x can be rearranged to have word2 as a prefix.
Return the total number of valid substrings of word1.
Example 1:
Input: word1 = "bcca", word2 = "abc"
Output: 1
Explanation:
The only valid substring is "bcca" which can be rearranged to "abcc" having "abc" as a prefix.
Example 2:
Input: word1 = "abcabc", word2 = "abc"
Output: 10
Explanation:
All the substrings except substrings of size 1 and size 2 are valid.
Example 3:
Input: word1 = "abcabc", word2 = "aaabc"
Output: 0
Constraints:
1 <= word1.length <= 1051 <= word2.length <= 104word1 and word2 consist only of lowercase English letters.