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

轉自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:

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

<Solution>

想法如下

• 這題並不難，主要就是檢查是不是質數。因為題目有規定範圍是 1 - 10^6，查了一下，只要檢查個數是不是 2,3,5,7,11,13,17,19 就可以了

code 如下

Java

	class Solution {
	public int countPrimeSetBits(int L, int R) {
	int ans = 0;
	for(int i = L; i <= R; i++) {
	if(isPrime(Integer.bitCount(i))) {
	++ans;
	}
	}
	return ans;
	}
	private boolean isPrime(final int num) {
	if(num == 2 || num == 3 || num == 5 ||
	num == 7 || num == 11 || num == 13 ||
	num == 17 || num == 19) {
	return true;
	}
	return false;
	}
	}

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