How should I practice Count Secured Strings?

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

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FastPrep

Problem · Dynamic Programming

Count Secured Strings

● HardAmazonFULLTIMEOA

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In an Amazon security analysis task, two passwords have been generated, but they may be differ in length. One password is generated by a customer, and the other by an internal system. The customer wants to determine how many secured variations of the passwords exist module 10^9 + 7.

An secured variation of the passwords is defined as a subsequence of customer's password which is lexicographically greater than system generated password.

Formally:

Person A has a password s (the customer's password)

Person B has a password t (the system-generated password)

The task is to count how many susequences of password s are lexicographically greater than password t. Since the answer can be large, return the result module (%) 10^9 + 7. More specifically, if result represents the required number of subsequences, then return the remainder when result is divided by 10^9 + 7.

Note

A subsequence is determined as a sequence derived from the original password by deleting zero or more characters without changing the order of the remaining characters. For example, "ac" is a susequence of "abc", but "ca" is not.

A sequence x is considered lexicographically greater than a sequence y if:

-->x[i] > y[i] at the first position where x and y differ. or

-->|x| > |y| and y is a prefix of x (where |x| denotes the length of password x).

💜 Thanks a jillion, spike! 💜

Examples

01 · Example 1

s = "aba"
t = "ab"
return = 3

From all posibble subsequences, 3 are lexicographically smaller, 1 is equal, and 3 are greater than t, hence the answer is 3 :)

02 · Example 2

s = "bab"
t = "ab"
return = 5

Lets examine all possible susequences that can be derived from s = "bab" and compare them lexicographically with t = "ab": The rest of the explanation is not complete, so I did not add them here :)

Constraints

1 <= |s| <= 10^5
1 <= |t| <= 100
s and t consists of lowercase English letters.