【Leetcode_easy】762. Prime Number of Set Bits in Binary Representation

problem

762. Prime Number of Set Bits in Binary Representation

solution1:

class Solution {
public:
    int countPrimeSetBits(int L, int R) {
        int res = 0;
        int bits = 0;
        for(int i=L; i<=R; i++)
        {
            bits = countBits(i);
            if(isPrime(bits) && bits!=1 ) res++;//err...
        }
        return res;
    }
    int countBits(int num)
    {
        int res = 0;
        while(num>0)
        {
            if(num & 1 == 1) res++;
            num >>= 1;
        }
        return res;
    }
    bool isPrime(int num)
    {
        for(int i=sqrt(num); i>1; i--)
        {
            if(num%i==0) return false;
        }
        return true;
 
    }
};

solution2:题目中给了数的大小范围 R <= 10⁶ < 2²⁰，那么统计出来的非零位个数cnt只需要检测是否是20以内的质数即可，所以将20以内的质数都放入一个HashSet中，然后统计出来cnt后，直接在HashSet中查找有没有即可。

class Solution {
public:
    int countPrimeSetBits(int L, int R) {
        int res = 0;
        unordered_set<int> primes{2, 3, 5, 7, 11, 13, 17, 19};
        for(int i=L; i<=R; i++)
        {
            int bits = 0;
            int tmp = i;
            while(tmp){
                if(tmp&1 == 1) bits++;
                tmp >>= 1;
            }
            res += primes.count(bits);
        }
        return res;
    }
};