Is Sum of Primes Between Number and Its Reverse easy or hard?

The problem is generally classified as Medium because it combines two ideas: reversing an integer and efficiently computing primes in a numeric range. Developers familiar with sieve techniques can implement the optimal solution quickly.

Sum of Primes Between Number and Its Reverse Python/Java solution

Both Python and Java implementations typically compute the reversed integer, run a Sieve of Eratosthenes up to the maximum value, and iterate through the interval to accumulate primes. The core logic is identical across languages: sieve construction followed by a linear scan.

How to solve Sum of Primes Between Number and Its Reverse in O(n)?

A near‑linear solution uses the Sieve of Eratosthenes. Build a prime sieve up to the larger value between the number and its reverse, which takes O(n log log n). Then iterate through the required range and accumulate primes using constant‑time lookups from the sieve array.

What is the best approach for Sum of Primes Between Number and Its Reverse?

The Sieve of Eratosthenes is the most practical solution. Compute the reversed value of the number, determine the range [L, R], generate primes up to R using a sieve, then sum primes within the interval. This avoids repeated primality checks and runs in O(R log log R) time.

Is Sum of Primes Between Number and Its Reverse asked at Google/Amazon/Meta?

Variants of prime range queries and number reversal problems appear in interviews at large tech companies. While the exact problem title may vary, companies like Amazon and Google frequently test number theory basics such as primality testing and sieve-based optimizations.

What data structure is used in Sum of Primes Between Number and Its Reverse?

The optimal approach uses a boolean array (or bitset) to represent a sieve of primes. Each index indicates whether the number is prime, enabling constant-time checks when summing primes within the computed range.

What is the time complexity of Sum of Primes Between Number and Its Reverse?

The brute force approach that checks primality for each number runs in about O(k * sqrt(k)), where k is the size of the interval. The optimized sieve solution runs in O(R log log R) time with O(R) space, which is significantly faster for larger ranges.

3918. Sum of Primes Between Number and Its Reverse - Practice Online

You are given an integer n.

Let r be the integer formed by reversing the digits of n.

Return the sum of all prime numbers between min(n, r) and max(n, r), inclusive.

Example 1:

Input: n = 13

Output: 132

Explanation:

The reverse of 13 is 31. Thus, the range is [13, 31].
The prime numbers in this range are 13, 17, 19, 23, 29, and 31.
The sum of these prime numbers is 13 + 17 + 19 + 23 + 29 + 31 = 132.

Example 2:

Input: n = 10

Output: 17

Explanation:

The reverse of 10 is 1. Thus, the range is [1, 10].
The prime numbers in this range are 2, 3, 5, and 7.
The sum of these prime numbers is 2 + 3 + 5 + 7 = 17.

Example 3:

Input: n = 8

Output: 0

Explanation:

The reverse of 8 is 8. Thus, the range is [8, 8].
There are no prime numbers in this range, so the sum is 0.

Constraints:

1 <= n <= 1000