Leetcode:count Primes-Statistics number of prime numbers

1. Title

Count Primes (count number of prime numbers)

2. Address of the topic

https://leetcode.com/problems/count-primes/

3. Topic content

English: Count The number of prime numbers less than a non-negative number, N.

English: Count the number of prime numbers within a positive integer n (excluding n itself)

4, a method of tle

From 1 to N, examine whether each number is prime. This method can not meet the requirement of time in the problem because it takes a long time.

A section of Java code that implements this method is as follows:

/** *  function Description:leetcode 204 - count primes *  developer:tsybius2014 *  Development Date: September 6, 2015  */public class solution {        /* *     *  calculates the number of prime numbers below n      *  @param  n  positive integers      *  @return Number of prime numbers below  n      */     public int countprimes (int n)  {                 if  (n <= 1)  {             return 0;         }                int  result = 0;        boolean isprime = true;         for  (int i = 2; i < n; i++)  {             //Judging whether the number I is prime              isPrime = true;             if  (i == 2 | |  i == 3 | |  i == 5 | | &NBSP;I&NBSP;==&NBSP;7)  {                 isPrime = true;             } else if  (i % 2 == 0 | |  i % 3 == 0 | |  i % 5 == 0 | |  i % 7 == 0)  {                 isprime = false;            } else {                 for  (int j = 2; j  &LT;=&NBSP;MATH.SQRT (i);  j++)  {                     if  (i % j == 0)  {                          isPrime = false;                         break;                     }                 }             }                         //If I is prime number result self-increment 1             if  (IsPrime)  {                 result++;             }        }                 return result;     }}

4. Methods of Solving problems

Another method for prime numbers is the Eratosthenes sieve method (Sieve of Eratosthenes), which needs to declare a very large array, but at a much faster speed than the above method.

A section of Java code that implements this method is as follows:

/** *  function Description:leetcode 204 - count primes *  developer:tsybius2014 *  Development Date: September 6, 2015  */public class solution {        /* *     *  calculates the number of prime numbers below n      *  @param  n  positive integers      *  @return Number of prime numbers below  n      */     public int countprimes (int n)  {                 if  (n <= 1)  {             return 0;         }        int result = 0;         boolean[] arr = new boolean[n];         for  (INT&NBsp;i = 2; i < n; i++)  {                         //if Arr[i] is a prime number that marks all multiples as composite, otherwise it is not considered             if  (!arr[i] )  {                result ++;            } else {                 continue;             }             int j = 2;             while  (i * j < n)  {                 arr[i * j] = true;                 j++;             }        }                 return result;    }}

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Leetcode:count Primes-Statistics number of prime numbers