输入一个大数（正的），输出它算数平方根的整数部分。还是java省心

import java.io.*;
import java.util.*;
import java.math.*;

public class Main {
	public static void main(String[] args){
		Scanner cin = new Scanner(System.in);
		BigInteger n, m;
		while(cin.hasNextBigInteger()){
			n = cin.nextBigInteger();
			String theNumber = n.toString();
			m = sqrt(theNumber);
			System.out.println(m);
		}
	}
	public static BigInteger sqrt(String theNumber) {   
        int length = theNumber.length(), i;   
        BigInteger res = BigInteger.ZERO;   
        BigInteger twenty = BigInteger.valueOf(20);   
        BigInteger t, x=BigInteger.ZERO, v, few=BigInteger.ZERO;   
        BigInteger hg = BigInteger.valueOf(100);   
  
        String tmpString = null;   
        int pos = 2-length%2;   
        tmpString = theNumber.substring(0, pos);   
        while (true) {   
            v = few.multiply(hg).add(BigInteger.valueOf(Integer.parseInt(tmpString)));   
            if (res.compareTo(BigInteger.ZERO)==0) i=9;   
            else i = v.divide(res.multiply(twenty)).intValue();   
            for (; i>=0; i--) {   
                t = res.multiply(twenty).add(BigInteger.valueOf(i)).multiply(BigInteger.valueOf(i));   
                if (t.compareTo(v)<=0) {   
                    x = t;   
                    break;   
                }   
            }   
            res = res.multiply(BigInteger.TEN).add(BigInteger.valueOf(i));   
            few = v.subtract(x);   
            pos++;   
            if (pos>length) break;   
            tmpString = theNumber.substring(pos-1, ++pos);   
        }   
        return res;   
    }   
}