[leetcode-762-Prime Number of Set Bits in Binary Representation]

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:

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

思路：

根据条件 考虑到 L到R这个区间里面的数字写成2进制，最多也就是20多位，可以事先将30以内的素数放到集合里面。

 int countPrimeSetBits(int L, int R)
 {
    set<int> primes = {2,3,5,7,11,13,17,19,23,29};
    int cnt = 0;
    for(int i=L;i<=R;i++)
    {
        int bits = 0;
        for(int n=i;n>0;n>>=1)
        {
            bits += n&1;
        }
        cnt += primes.count(bits);        
    }    
    return cnt;
 }

参考：

https://discuss.leetcode.com/topic/117013/java-c-clean-code