BOJ 13706. 제곱근

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

public class Main {
	public static void main(String[] args) throws Exception {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		BigInteger dest = new BigInteger(in.readLine());

		BigInteger two = BigInteger.valueOf(2);
		BigInteger left = BigInteger.valueOf(1);
		BigInteger right = dest;
		while (left.compareTo(right) <= 0) {
			BigInteger mid = left.add(right).divide(two);
			BigInteger square = mid.multiply(mid);
			if (square.equals(dest)) {
				System.out.println(mid);
				break;
			} else if (square.compareTo(dest) < 0)
				left = mid.add(BigInteger.ONE);
			else
				right = mid.subtract(BigInteger.ONE);
		}
	}
}