素数及质因数分解

import java.util.HashMap;
import java.util.Map;

public class PrimeNumber {

	public static void main(String[] args) {
		boolean res = isPrime(7);
		System.out.println(res);
		Map<Integer, Integer> map = primeFactor(100);
		StringBuilder sb = new StringBuilder();
		for(Map.Entry<Integer, Integer> entry:map.entrySet()){
			int k = entry.getKey();
			int v = entry.getValue();
			for (int i = 0; i < v; i++) {
				sb.append("*"+k);
			}
		}
		System.out.println(map);
		System.out.println(sb.substring(1));
	}
	
	/**
	 * 检查num是不是素数
	 * 2~根号n
	 */
	public static boolean isPrime(long num){
		for (int i = 2; i*i <= num; i++) {
			if (num%i==0) {
				return false;
			}
		}
		return true;
	}
	
	/**
	 * 质因素分解：100 = 2*2*5*5
	 * map是质因数-出现次数的映射
	 */
	public static Map<Integer, Integer> primeFactor(int num){
		Map<Integer, Integer> map = new HashMap<>();
		for (int i = 2; i*i <= num; i++) {
			while(num%i==0){
				Integer v = map.get(i);
				if (v==null) {	
					map.put(i, 1);
				}else {
					map.put(i, v+1);
				}
				num /= i;
			}
		}
		return map;
	}

}