Leetcode: Count Primes
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Description:
Count the number of prime numbers less than a non-negative number, n.
数学的东西,有公式。最直观的检查到开方。还有很著名的算法:2的倍数,3的倍数,最大到开方一直检查下去。
class Solution {public: int countPrimes(int n) { vector<bool> primes(n, true); for (int i = 2; i * i < n; ++i) { if (!primes[i]) { continue; } for (int j = i * i; j < n; j += i) { primes[j] = false; } } int count = 0; for (int i = 2; i < n; ++i) { if (primes[i]) { ++count; } } return count; }};
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