ARTS打卡计划第5周-ALGORITHM

168. Excel表列名称

public class Solution168 {
	 public String convertToTitle(int n) {
		String restult = "";
		 while(n>0) {
			 n--;
			 restult =(char)('A'+(n%26))+restult;
			 n=n/26;
			 
		 }
		return restult;
	        
	    }
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Solution168 s = new Solution168();
		System.err.println(s.convertToTitle(26));
		System.err.println(s.convertToTitle(52));
		System.err.println(s.convertToTitle(27));
		System.err.println(s.convertToTitle(26));
		System.err.println(s.convertToTitle(28));
		System.err.println(s.convertToTitle(701));
	}

}

　　

204. 计数质数

public class Solution204 {
	public int countPrimes(int n) {
		int result = 0;
		for (int i = 1; i < n; i++) {
			if (isPrime(i)) {
				result++;
			}
		}
		return result;
	}

	/**
	 * AKS算法计算一个数是否是素数
	 * https://stackoverflow.com/questions/1801391/what-is-the-best-algorithm-for-checking-if-a-number-is-prime
	 * @param n
	 * @return
	 */
	public boolean isPrime(int n) {
		if (n == 1) {
			return false;
		}
		if (n == 2 || n == 3) {
			return true;
		}
		if (n % 2 == 0 || n % 3 == 0) {
			return false;
		}
		int i = 5, w = 2;
		while (i * i <= n) {
			if (n % i == 0) {
				return false;
			}
			i += w;
			w = 6 - w;
		}

		return true;

	}

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Solution204 s = new Solution204();
		System.out.println(s.countPrimes(10));
	}

}