标签：family IV top tip turn amp prim pre col
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.
Solution2: ssieving: need a helping array with false initialization: false(prime) true(non- prime), inner loop: only set the multiple of prime.
class Solution { int res = 0; //solution 1: fun: check each unmber n * (n) //seieve solution: 2,3,5,7,11,13 public int countPrimes(int n) { // 2: 1 3: 2 4: 2 ,5 :3 .... boolean[] aux = new boolean[n+1]; //all false (prime) for(int i = 2; i <n; i++){//i: number in n if(aux[i] == false) res++; if(aux[i] == true) continue; //pass the non -prime question: 2 and 4 have the same multiple?? //sieving the number is multiple of this target nuber : aux[i] for(int j = 2; j*i <n; j++){ aux[j*i] = true; } } return res; } //time: i = 2 ：n/2 // i = 3 : n/3 or smaller // sum(n/i) : i is prime : n*sum(1/i) = n*(lg(lgn)) }
Solution 2: is prime function
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.
*204. Count Primes (siecing prime)
标签：family IV top tip turn amp prim pre col
原文地址：https://www.cnblogs.com/stiles/p/leetcode204.html
