Count Primes Below N
Return how many prime numbers are strictly less than n.
Example 1
Input: n = 10 Output: 4 Explanation: The primes below 10 are 2, 3, 5 and 7.
Example 2
Input: n = 2 Output: 0
Constraints
- 0 <= n <= 2 * 10^6
Hint 1
Use the Sieve of Eratosthenes.
Hint 2
Start crossing out multiples of p at p * p.
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