獲取n以內素數列表
使用篩選法,生成正整數n以內素數列表。
算法描述:
初始設置BitSet從0到n的值均爲true。
從2開始,由於2是素數,所以將所有2的倍數排除;然後下一個素數是3,則將所有3的倍數排除;下一個素數是
5,將所有5的倍數排除…
以此類推,直到n。
BitSet中剩餘的值爲true的index即爲素數。
具體代碼:
/**
* 篩選法生成正整數n以內素數列表
* 描述:
* 初始設置BitSet從0到n的值均爲true
* 從2開始,由於2是素數,所以將所有2的倍數排除
* 然後下一個素數是3,則將所有3的倍數排除
* 下一個素數是5,將所有5的倍數排除
* ...
* 直到n
* BitSet中剩餘的值爲true的index即爲素數
*
* @param n 正整數
* @return 素數表,用 BitSet 表示
*/
public static BitSet genPrimeBitSet(int n) {
BitSet primeBitSet = new BitSet(n);
primeBitSet.set(0, n, true);
primeBitSet.set(0, false);
primeBitSet.set(1, false);
for (int i = 2; i <= n; i++) {
if (primeBitSet.get(i)) {
for (int j = 2 * i; j <= n; j += i) {
primeBitSet.set(j, false);
}
}
}
return primeBitSet;
}
測試代碼:
public static void main(String[] args) {
// 獲取n以內的所有素數
int n = 1000;
long begin = System.currentTimeMillis();
BitSet primeBitSet = genPrimeBitSet(n);
StringBuilder sb = new StringBuilder();
int num = 0;
for (int i = 0; i <= n; i++) {
if (primeBitSet.get(i)) {
sb.append(i).append(",");
num++;
}
}
long end = System.currentTimeMillis();
sb.delete(sb.toString().length() - 1, sb.toString().length());
String desc = n + " 以內共 " + num + " 個素數,耗時 " + (end - begin) + "毫秒";
sb.append(System.lineSeparator()).append(desc);
System.out.println(sb.toString());
}
以上代碼輸出爲:
2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,73,79,83,89,97,101,103,107,109,113,127,131,137,139,149,151,157,163,167,173,179,181,191,193,197,199,211,223,227,229,233,239,241,251,257,263,269,271,277,281,283,293,307,311,313,317,331,337,347,349,353,359,367,373,379,383,389,397,401,409,419,421,431,433,439,443,449,457,461,463,467,479,487,491,499,503,509,521,523,541,547,557,563,569,571,577,587,593,599,601,607,613,617,619,631,641,643,647,653,659,661,673,677,683,691,701,709,719,727,733,739,743,751,757,761,769,773,787,797,809,811,821,823,827,829,839,853,857,859,863,877,881,883,887,907,911,919,929,937,941,947,953,967,971,977,983,991,997
1000 以內共 168 個素數,耗時 3毫秒