LeetCode 204. Count Primes
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解题思路:维基百科中 https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
Input: an integer n > 1. Let A be an array of Boolean values, indexed by integers 2 to n, initially all set to true. for i = 2, 3, 4, ..., not exceeding √n: if A[i] is true: for j = i2, i2+i, i2+2i, i2+3i, ..., not exceeding n: A[j] := false. Output: all i such that A[i] is true.
class Solution {public: int countPrimes(int n) { if (2 >= n) return 0; vector<bool>primes(n,true); int sqr = sqrt(n - 1); for (int i = 2; i <= sqr; ++i){ if (primes[i]){ for (int j = i * i; j < n; j += i) primes[j] = false; } } int sum = 0; for (int i = 2; i < n; ++i) sum += (primes[i]) ? 1 : 0; return sum; }};
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