Prime number

思路： 利用 prime 的倍数 一定不是prime ， 来进行线性的剔除。

易错点：刚开始时候要先把全部初始化为true, 获取下一个prime 时候 一定要先 prime++;

package PrimeNumber;

public class Solution {

	public boolean[] getPrime(int max) {
		boolean[] ret = new boolean[max + 1];
		if (max < 1)
			return ret;
		for (int i = 0; i <= max; i++) {
			ret[i] = true;
		}
		int prime = 2;
		while (prime <= max) {
			crossOff(ret, prime);
			prime++;
			while (prime <= max && !ret[prime]) {
				prime++;
			}
		}
		return ret;
	}

	private void crossOff(boolean[] ret, int prime) {
		for (int i = prime * prime; i < ret.length; i += prime) {
			ret[i] = false;
		}
	}

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Solution solution = new Solution();
		boolean[] ret = solution.getPrime(20);
		for (int i = 0; i <= 20; i++) {
			if (ret[i])
				System.out.print(i + ", ");
		}
	}

}