How should I practice Count Numbers with a Prime Digit Sum?

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

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Count Numbers with a Prime Digit Sum

Rippling● HardINTERNFULLTIMEOA

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Problem statement

lower and upper are canonical decimal strings representing positive integers L and R. Count the integers x in the inclusive interval [L, R] whose decimal digit sum is prime.

A prime is an integer greater than one with exactly two positive divisors. Return the count as a canonical decimal string so that large inputs and results remain exact.

Function

countPrimeDigitSums(lower: String, upper: String) → String

Examples

Example 1

lower = "1"upper = "20"return = "9"

The qualifying numbers are 2, 3, 5, 7, 11, 12, 14, 16, and 20.

Example 2

lower = "98"upper = "102"return = "3"

The qualifying numbers are 98, 101, and 102, with digit sums 17, 2, and 3.

Constraints

1 <= L <= R <= 10^18.
Input strings contain only decimal digits and have no leading zeros.
Use exact integer arithmetic; no modulo is applied.