How should I practice String Subsequences 🐳?

Start by reading the full statement, solve the visible examples, then compare the pattern against related Morgan Stanley online assessment problems in FastPrep.

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String Subsequences 🐳

Morgan Stanley● EasyINTERNOA

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Problem statement

Given two strings, determine the number of times the first one appears as a subsequence in the second one. A subsequence is created by eliminating any number of characters from a string (possibly 0) without changing the order of the characters retained.

Function

getSubsequenceCount(s1: String, s2: String) → long

Complete the function getSubsequenceCount in the editor below.

getSubsequenceCount has the following parameters:

string s1: the first string, which always has a length of 3
string s2: the second string

Returns int: the number of times s1 appears as a subsequence in s2

Examples

Example 1

s1 = "HRW"s2 = "HERHRWS"return = 3

HRW appears at 1-based positions (1, 3, 6), (1, 5, 6), and (4, 5, 6), so the count is 3.

Example 2

s1 = "ELO"s2 = "HELLOWORLD"return = 4

ELO appears at 1-based positions (2, 3, 5), (2, 3, 7), (2, 4, 5), and (2, 4, 7), so the count is 4.

Example 3

s1 = "ABC"s2 = "ABCBABC"return = 5

ABC appears at 1-based positions (1, 2, 3), (1, 2, 7), (1, 4, 7), (1, 6, 7), and (5, 6, 7), so the count is 5.

Constraints

length of s1 = 3
1 ≤ length of s2 ≤ 5*105
s1 and s2 consist of uppercase English letters, A-Z.