Given a number N which is a prime number, the task is to find the position of the given prime number in the series of Prime Numbers.
Input: N = 11
The prime numbers are 2, 3, 5, 7, 11, 13, 17, ….
Therefore, the position of 11 in this series is 5.
Input: N = 13
Naive Approach: The naive approach for this problem is for the given input, compute the prime numbers which are less than that number and keep a track of the number of primes less than the given N. If the count is K, then K + 1 would be the answer. The time complexity for this approach is quadratic.
Efficient Approach: The idea is to use the slight modification of Sieve of Eratosthenes. All the prime numbers up to the maximum value can be computed and stored in an array along with its position. Clearly, when the prime numbers are stored in an array, the index at which the number is stored is the position of the number in the series. After this precomputation, the answer can be calculated in constant time.
Below is the implementation of the above approach:
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