原文:
给定一个数 n，任务是如果这个数不是素数，则打印最近的素数，方法是从 2 开始依次添加素数，使其成为素数。 例:
输入: n = 8 输出: 13 8 不是素数，所以加上第一个素数得到 10 10 不是素数，因此加上第二个素数，即 3 得到 13，即素数。 输入: n = 45 输出: 47
逼近使用厄拉多塞的筛，在is prime【】列表中用 1 标记质数索引，并将所有质数存储在一个列表prime【】中。继续把质数按顺序加到 n 上，直到它变成质数。 以下是上述方法的实施:
c
// c program to print the
// nearest prime number by
// sequentially adding the
// prime numbers
#include
using namespace std;
// function to store prime
// numbers using prime sieve
void prime_sieve(int max, vector &isprime,
vector &prime)
{
// iterate for all
// the numbers
int i = 2;
while (i * i <= max)
{
// if prime[p] is not changed,
// then it is a prime
if (isprime[i] == 1)
{
// append the prime
// to the list
prime.push_back(i);
// update all multiples of p
for (int j = i * 2; j < max; j = i)
{
isprime[j] = 0;
}
}
i = 1;
}
}
// function to print
// the nearest prime
int printnearest(int n)
{
int max = 1e6;
// store all the
// index with 1
vector isprime(max, 1);
// 0 and 1 are not prime
isprime[0] = isprime[1] = 0;
// list to store
// prime numbers
vector prime;
// variable to
// add primes
int i = 0;
// call the sieve function
prime_sieve(max, isprime, prime);
// keep on adding prime
// numbers till the nearest
// prime number is achieved
while (!isprime[n])
{
n = prime[i];
i = 1;
}
// return the
// nearest prime
return n ;
}
// driver code
int main()
{
int n = 8;
printf("%d", printnearest(n));
return 0;
}
// this code is contributed
// by harshit saini
java 语言(一种计算机语言，尤用于创建网站)
// java program to print the
// nearest prime number by
// sequentially adding the
// prime numbers
import java.util.*;
class gfg
{
// function to store prime
// numbers using prime sieve
static void prime_sieve(int max, int []isprime,
vector prime)
{
// iterate for all
// the numbers
int i = 2;
while (i * i <= max)
{
// if prime[p] is not changed,
// then it is a prime
if (isprime[i] == 1)
{
// append the prime
// to the list
prime.add(i);
// update all multiples of p
for (int j = i * 2;
j < max; j = i)
{
isprime[j] = 0;
}
}
i = 1;
}
}
// function to print
// the nearest prime
static int printnearest(int n)
{
int max = (int) 1e6;
// store all the
// index with 1 except 0,1 index
int [] isprime = new int[max];
for(int i = 2; i < max; i )
isprime[i] = 1;
// list to store
// prime numbers
vector prime = new vector();
// variable to add primes
int i = 0;
// call the sieve function
prime_sieve(max, isprime, prime);
// keep on adding prime
// numbers till the nearest
// prime number is achieved
while (isprime[n] == 0)
{
n = prime.get(i);
i = 1;
}
// return the
// nearest prime
return n ;
}
// driver code
public static void main(string[] args)
{
int n = 8;
system.out.printf("%d", printnearest(n));
}
}
// this code is contributed by rajput-ji
python 3
# python3 program to print the nearest prime
# number by sequentially adding the prime numbers
# function to store prime numbers using prime sieve
def prime_sieve(max, isprime, prime):
# iterate for all the numbers
i = 2
while (i * i <= max):
# if prime[p] is not changed,
# then it is a prime
if (isprime[i] == 1):
# append the prime to the list
prime.append(i)
# update all multiples of p
for j in range(i * 2, max, i):
isprime[j] = 0
i = 1
# function to print the nearest prime
def printnearest(n):
max = 10**6
# store all the index with 1
isprime = [1] * max
# 0 and 1 are not prime
isprime[0] = isprime[1] = 0
# list to store prime numbers
prime = []
# variable to add primes
i = 0
# call the sieve function
prime_sieve(max, isprime, prime)
# keep on adding prime numbers
# till the nearest prime number
# is achieved
while not isprime[n]:
n = prime[i]
i = 1
# return the nearest prime
return n
# driver code
n = 8
print(printnearest(n))
c
// c# program to print the
// nearest prime number by
// sequentially adding the
// prime numbers
using system;
using system.collections.generic;
class gfg
{
// function to store prime
// numbers using prime sieve
static void prime_sieve(int max, int []isprime,
list prime)
{
// iterate for all the numbers
int i = 2;
while (i * i <= max)
{
// if prime[p] is not changed,
// then it is a prime
if (isprime[i] == 1)
{
// append the prime to the list
prime.add(i);
// update all multiples of p
for (int j = i * 2;
j < max; j = i)
{
isprime[j] = 0;
}
}
i = 1;
}
}
// function to print
// the nearest prime
static int printnearest(int n)
{
int max = (int) 1e6;
int i = 0;
// store all the
// index with 1 except 0,1 index
int [] isprime = new int[max];
for(i = 2; i < max; i )
isprime[i] = 1;
// list to store
// prime numbers
list prime = new list();
// variable to add primes
i = 0;
// call the sieve function
prime_sieve(max, isprime, prime);
// keep on adding prime
// numbers till the nearest
// prime number is achieved
while (isprime[n] == 0)
{
n = prime[i];
i = 1;
}
// return the
// nearest prime
return n;
}
// driver code
public static void main(string[] args)
{
int n = 8;
console.write("{0}", printnearest(n));
}
}
// this code is contributed by princi singh
java 描述语言
output:
13
时间复杂度:o(n * log(logn)) t3】辅助空间: o(n)
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