204. Count Primes (Integer)
Count the number of prime numbers less than a non-negative number, n.
思路:寻找质数的方法
class Solution { public: int countPrimes(int n) { int num = 0; if(n < 2) return num; int i, j, index; isPrime = new bool [n]; isPrime[0]=false; isPrime[1]=false; for(i = 2; i < n; i++){ isPrime[i] = true;//initialize as true, means all are primes } for(i = 2; i < n; i++){ if(!isPrime[i]) continue; num++; for(j=2; i*j < n; j++){ isPrime[i*j] = false; } } return num; } private: bool* isPrime; };