Given two strings s and t, return the number of distinct subsequences of s which equals t. A string's subsequence is a new string formed from the original string by deleting some (can be none) of the characters without disturbing the remaining characters' relative positions. (i.e., "ACE" is a subsequence of "ABCDE" while "AEC" is not). The test cases are generated so that the answer fits on a 32-bit signed integer.