LeetCode 762. 762. Prime Number of Set Bits in Binary Representation

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题目链接：二进制表示中质数个计算置位 - 力扣 (LeetCode)

Given two integers L and R, find the count of numbers in the range [L, R] (inclusive) having a prime number of set bits in their binary representation.

(Recall that the number of set bits an integer has is the number of 1s present when written in binary. For example, 21 written in binary is 10101 which has 3 set bits. Also, 1 is not a prime.)

Example 1:

Input: L = 6, R = 10
Output: 4
Explanation:
6 -> 110 (2 set bits, 2 is prime)
7 -> 111 (3 set bits, 3 is prime)
9 -> 1001 (2 set bits , 2 is prime)
10->1010 (2 set bits , 2 is prime)

Example 2:

Input: L = 10, R = 15
Output: 5
Explanation:
10 -> 1010 (2 set bits, 2 is prime)
11 -> 1011 (3 set bits, 3 is prime)
12 -> 1100 (2 set bits, 2 is prime)
13 -> 1101 (3 set bits, 3 is prime)
14 -> 1110 (3 set bits, 3 is prime)
15 -> 1111 (4 set bits, 4 is not prime)

Note:

• L, R will be integers L <= R in the range [1, 10^6].
• R - L will be at most 10000.

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本题有个窍门！！
2 ^ 20 = 1024 ^ 2 > 10 ^ 6
所以L , R的二进制表示最多有20位。
所以只需要模拟出来20以内素数就ok！

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