Java计算平方根算法

public class SqrtTest{
	public static double mySqrt(double num, double offSet){
		if(num <= 0) return 0;
		double flag = 10.0;
		int counter = 0;
		do{
			if (Math.abs(flag*flag - num) < offSet) {
				System.out.println(flag);
				return flag;
			}else{
				System.out.println(flag);
				counter++;
				flag = (flag + num/flag)/2;
			}
		}while(counter<100);
		return -1;
	}
	public static void main(String[] args){
		System.out.println(mySqrt(12545, 0.2));
	}
}

上述方法是g=(g+x/g)的方法。若不知道该方法，亦可用二分法来逼近。是偶数次开方时，初始下界为0，上初始界为x；奇数次开方时，初始下界为-|x|，上初始界为|x|。后续二分查找直至逼近或者到达搜索次数退出。

public class MySqrt{
	public static double doMySqrt(double num, double offSet, int sqrtNum){
		double low = 0, high = 0;
		if(sqrtNum <= 0){
			 return 0;
		}else if(sqrtNum % 2 == 0){
			high = num;
		}else{
			low = -Math.abs(num);
			high = Math.abs(num);
		}
		int counter = 0;
		double flag = (low + high)/2;
		do{
			if (Math.abs(Math.pow(flag, sqrtNum)-num)<offSet) return flag;
			if(Math.pow(flag, sqrtNum)>num){
				 high = flag;
			}else{
				 low = flag;
			}
			flag =  (low + high)/2;
			counter ++;
		}while(counter < 100);
		return 0;
	}
	public static void main(String[] args){
		System.out.println(doMySqrt(-45, 0.1, 3));
	}
}