我们可以发现，a 和 b 的数据范围特别大，居然超过了 long long！我们只好拿出秘密武器：高精度。我们可以发现 (a+b)2=a2+b2+2ab（完全平方公式）。所以可以使用**高精度乘法算出来 a2,b2,2ab，再使用高精度开根算出绝对值 a+b**。这时候我们就需要分类讨论。若 a 为正数且 b 为正数，则 a+b 为正数。若 a 为负数且 b 为负数，则 a+b 为负数。若 a 与 b 一正一负，则要比较 a 与 b 的绝对值，用到高精度比较。

来人，把朕的代码端上来！！！

#include <bits/stdc++.h>
using namespace std;
string qf (string a)
{
	if (a[0] == '-') return a.substr (1 , a.size () - 1);
	return "-" + a;
}
string operator - (string a) {return qf (a);}
string gjc (string a , string b)
{
	if (a[0] == '-' && b[0] == '-') return gjc (- a , - b);
	else if (a[0] == '-') return - gjc (- a , b);
	else if (b[0] == '-') return - gjc (a , - b);
	int ans[a.size () + b.size ()] = {};
	reverse (a.begin () , a.end ());
	reverse (b.begin () , b.end ());
	for (int i = 0;i < a.size ();i ++)
		for (int j = 0;j < b.size ();j ++) ans[i + j] += (a[i] - '0') * (b[j] - '0');
	for (int i = 0;i < a.size () + b.size ();i ++)
		if (ans[i] > 9)
		{
			int x = ans[i] % 10 , y = ans[i] / 10;
			ans[i] = x;
			ans[i + 1] += y;
		}
	string ans2 = "";
	for (int i = 1;i <= a.size () + b.size ();i ++) ans2 += (char) (ans[i - 1] + '0');
	if (ans2[ans2.size () - 1] == '0') ans2 = ans2.substr (0 , ans2.size () - 1);
	reverse (ans2.begin () , ans2.end ());
	return ans2;
}
string max (string a , string b)
{
	if (a[0] == '-' && b[0] == '-')
	{
		if (- max (- a , - b) == a) return b;
		return a;
	}
	else if (a[0] == '-') return b;
	else if (b[0] == '-') return a;
	else if (a.size () > b.size ()) return a;
	else if (a.size () < b.size ()) return b;
	else
	{
		for (int i = 0;i < a.size ();i ++)
			if (a[i] > b[i]) return a;
			else if (a[i] < b[i]) return b;
		return a;
	}
}
string min (string a , string b)
{
	if (max (a , b) == a) return b;
	return a;
}
string gjj (string a , string b);
string cut (string a , string b)
{
	if (a[0] == '-' && b[0] == '-') return cut (- b , - a);
	else if (a[0] == '-') return - gjj (- a , b);
	else if (b[0] == '-') return gjj (b , - a);
	else if (max (a , b) != a) return - cut (b , a);
	reverse (a.begin () , a.end ());
	reverse (b.begin () , b.end ());
	string ans = "";
	for (int i = 1;i <= a.size () + 1;i ++) ans += "0";
	for (int i = 0;i < b.size ();i ++) ans[i] += a[i] - b[i];
	for (int i = b.size ();i < a.size ();i ++) ans[i] = a[i];
	for (int i = 0;i < ans.size ();i ++)
		if (ans[i] < '0')
		{
			ans[i] += 10;
			ans[i + 1] -= 1;
		}
	while (ans.size () > 1 && ans[ans.size () - 1] == '0') ans = ans.substr (0 , ans.size () - 1);
	reverse (ans.begin () , ans.end ());
	return ans;
}
string gjj (string a , string b)
{
	if (a[0] == '-' && b[0] == '-') return - gjc (- a , - b);
	else if (a[0] == '-') return cut (b , - a);
	else if (b[0] == '-') return cut (a , - b);
	if (a.size () < b.size ()) swap (a , b);
	reverse (a.begin () , a.end ());
	reverse (b.begin () , b.end ());
	string ans = "";
	for (int i = 1;i <= a.size () + 1;i ++) ans += "0";
	for (int i = 0;i < b.size ();i ++) ans[i] += a[i] - '0' + b[i] - '0';
	for (int i = b.size ();i < a.size ();i ++) ans[i] = a[i];
	for (int i = 0;i < ans.size ();i ++)
		if (ans[i] > '9')
		{
			ans[i] -= 10;
			ans[i + 1] += 1;
		}
	if (ans[ans.size () - 1] == '0') ans = ans.substr (0 , ans.size () - 1);
	reverse (ans.begin () , ans.end ());
	return ans;
}
string dev (string b , int a)
{
	if (a < 0 && b[0] == '-') return dev (- b , - a);
	else if (a < 0) return - dev (b , - a);
	else if (b[0] == '-') return - dev (- b , a);
	while (a % 10 == 0 && b[b.size () - 1] == '0')
    {
        a /= 10;
        b = b.substr (0 , b.size () - 1);
    }
	string ans = "";
	int yu = 0;
	for (int i = 0;i < b.size ();i ++)
	{
		yu = yu * 10 + b[i] - '0';
		ans += yu / a + '0';
		yu %= a;
	}
	while (ans.size () > 1 && ans[0] == '0') ans = ans.substr (1);
	return ans;
}
string operator / (string a , int b) {return dev (a , b);}
string operator * (string a , string b) {return gjc (a , b);}
string operator + (string a , string b) {return gjj (a , b);}
string operator - (string a , string b) {return cut (a , b);}
string sqrt (string s)
{
	if (s == "0") return "0";
	if (s == "1") return "1";
	string l = "1" , r = "1" , ans , dan = "1";
	for (int i = 1;i < s.size () / 2;i ++) l += "0";
	for (int i = 1;i <= s.size () / 2 + 1;i ++) r += "0";
	while (max (l , r) == r)
	{
		string mid = (l + r) / 2;
		if (max (mid * mid , s) == s)
		{
			ans = mid;
			l = gjj (mid , dan);
		}
		else r = mid - dan;
	}
	return ans;
}
signed main ()
{
	string a , b , c = "2";
	cin >> a >> b;
	if (a[0] == '-' && b[0] == '-')	cout << - sqrt (a * a + b * b + c * a * b);
	else if (a[0] == '-')
	{
		if (max (- a , b) == b) cout << sqrt (a * a + b * b + c * a * b);
		else cout << - sqrt (a * a + b * b + c * a * b);
	}
	else if (b[0] == '-')
	{
		if (max (- b , a) == a) cout << sqrt (a * a + b * b + c * a * b);
		else cout << - sqrt (a * a + b * b + c * a * b);
	}
	else cout << sqrt (a * a + b * b + c * a * b);
}

哔~~