Count Primes(leetcode204)

2019独角兽企业重金招聘Python工程师标准>>>

Count the number of prime numbers less than a non-negative number, n.

Example:

Input: 10
Output: 4
Explanation: There are 4 prime numbers less than 10, they are 2, 3, 5, 7.

//想先用粗暴的方法实现一下
public static int countPrimes(int n) {int count = 0;for(int i = 1;i<n;i++){if(isPrime(i)){count ++;}}return count;}//全部判断一下是否书素数 但是这看到有是那个循环 时间复杂度接受不了
private static boolean isPrime(int i) {if(i ==1){return false;}for(int j =2;j< i;j++){for(int j2 =2;j2< i;j2++){if(j*j2 == i){return false;}}}return true;}//如果稍微优化一下呢 结果还是超时
public static boolean isPrime2(int i) {if(i ==1 ){return false;}//这样是否可以快点去掉一些数据if(i > 10 && (i%2==0 ||i%3==0||i%5==0||i%7==0)){return false;}for(int j =2;j< Math.sqrt(i)+1;j++){int j2 = (int)(Math.sqrt(i)) > 2?  (int)(Math.sqrt(i)):2;for(; j2< i; j2++){if(j*j2 == i){return false;}}}return true;}/**这里的想法是反一下 找出所有的合数*/
public static int countPrimes2(int n) {boolean[] notPrime = new boolean[n];int count = 0;for (int i = 2; i < n; i++) {if (notPrime[i] == false) {count++;for (int j = 2; i*j < n; j++) {notPrime[i*j] = true;}}}return count;
}//这里用了不断累加的方法 感觉也不错
public static  int countPrimes3(int n) {boolean[] m = new boolean[n];int count = 0;for (int i=2; i<n; i++) {if (m[i]) {continue;}count++;for (int j=i; j<n; j=j+i) {m[j] = true;}}return count;}

git：https://github.com/woshiyexinjie/leetcode-xin/tree/master/src/main/java/com/helloxin/leetcode/algorithms

转载于:https://my.oschina.net/u/2277632/blog/2962468

Count Primes(leetcode204)相关推荐

Leetcode-204 Count Primes
#204 Count Primes Count the number of prime numbers less than a non-negative number, n. 题解:这道题如果对每个小 ...
204. Count Primes
Count the number of prime numbers less than a non-negative number, n. Example: Input: 10 Output: 4 E ...
leetCode 204. Count Primes 哈希求素数
204. Count Primes 求素数 Description: Count the number of prime numbers less than a non-negative number ...
【LeetCode 剑指offer刷题】特殊数题3：204 Count Primes
[LeetCode & 剑指offer 刷题笔记]目录(持续更新中...) 204. Count Primes Count the number of prime numbers less t ...
[LeetCode] 204. Count Primes
204. Count Primes Count the number of prime numbers less than a non-negative number, n. Example: Inp ...
LeetCode204——count primes
方法还是有很多的,通过率很低,不过这题还是需要思考清楚,不然很容易t class Solution { public:int countPrimes(int n){if(n == 0 || n == ...
[LeetCode] Count Primes
Description: Count the number of prime numbers less than a non-negative number, n 解题思路采用Eratosthene ...
LeetCode Count Primes
Description: Count the number of prime numbers less than a non-negative number, n. 题意:给出一个数n,求小于n的素数 ...
[LeetCode] Count Primes - 素数系列问题
题目概述: Description: Count the number of prime numbers less than a non-negative number, n. 解题方法: 题意是给出 ...

Count Primes(leetcode204)

Count Primes(leetcode204)相关推荐

最新文章

热门文章