高精度模板

// C = A + B, A >= 0, B >= 0
vector add(vector &A, vector &B)
{
    if (A.size() < B.size()) return add(B, A);

    vector C;
    int t = 0;
    for (int i = 0; i < A.size(); i ++ )
    {
        t += A[i];
        if (i < B.size()) t += B[i];
        C.push_back(t % 10);
        t /= 10;
    }

    if (t) C.push_back(t);
    return C;
}

  public static List add(String a,String b){
        int i=a.length()-1;
        int j=b.length()-1;
        List temp=new ArrayList<>();
        int t=0;//用来进位的
        for(;i>=0||j>=0;i--,j--){//从个位数开始相加
            t+=(i>=0?a.charAt(i)-'0':0);//i超出了a的位数就取零
            t+=(j>=0?b.charAt(j)-'0':0);
            temp.add(t%10);
            t=t/10;
        }
        if(t>0) temp.add(t);//如果最后t==1就加上
        return temp;//此时的temp是逆序的，最后要在方法外逆序输出
    }

// C = A - B, 满足A >= B, A >= 0, B >= 0
vector sub(vector &A, vector &B)
{
    vector C;
    for (int i = 0, t = 0; i < A.size(); i ++ )
    {
        t = A[i] - t;
        if (i < B.size()) t -= B[i];
        C.push_back((t + 10) % 10);
        if (t < 0) t = 1;
        else t = 0;
    }

    while (C.size() > 1 && C.back() == 0) C.pop_back();
    return C;
}

import java.math.BigInteger;
import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        BigInteger a = sc.nextBigInteger();
        BigInteger b = sc.nextBigInteger();
        BigInteger cnt = a.subtract(b);
        System.out.println(cnt);
    }
}

import java.util.ArrayList;
import java.util.Scanner;

public class Main {
    //判断A和B的大小
    public static boolean cmp(ArrayList A,ArrayList B){
        if (A.size()!=B.size()) return A.size()>B.size();
        for (int i = A.size() - 1; i >= 0; i -- )
            if (A.get(i) != B.get(i))
                return A.get(i) > B.get(i);
        return true;
    }
    public static void main(String[] args) {
        Scanner scanner=new Scanner(System.in);
        String a,b;
        ArrayList A=new ArrayList<>();
        ArrayList B=new ArrayList<>();
        a=scanner.next();
        b=scanner.next();
        for (int i = a.length() - 1; i >= 0; i -- ) A.add(a.charAt(i) - '0');
        for (int i = b.length() - 1; i >= 0; i -- ) B.add(b.charAt(i) - '0');
        ArrayList C=null;
        if (cmp(A,B)) {//判断A和B的大小
            C=sub(A,B);//A大于B
        }else
        {
            C=sub(B,A);
            System.out.print("-");
        }
        for (int i = C.size() - 1; i >= 0; i -- )
            System.out.print(C.get(i));
        System.out.println();
    }
       //相减操作
    private static ArrayList sub(ArrayList A, ArrayList B) {
        ArrayList C=new ArrayList<>();
        for (int i = 0,t=0; i 1&&C.get(C.size()-1)==0)C.remove(C.size()-1);
        return C;
    }



}

// C = A * b, A >= 0, b >= 0
vector mul(vector &A, int b)
{
    vector C;

    int t = 0;
    for (int i = 0; i < A.size() || t; i ++ )
    {
        if (i < A.size()) t += A[i] * b;
        C.push_back(t % 10);
        t /= 10;
    }

    while (C.size() > 1 && C.back() == 0) C.pop_back();

    return C;
}

// A / b = C ... r, A >= 0, b > 0
vector div(vector &A, int b, int &r)
{
    vector C;
    r = 0;
    for (int i = A.size() - 1; i >= 0; i -- )
    {
        r = r * 10 + A[i];
        C.push_back(r / b);
        r %= b;
    }
    reverse(C.begin(), C.end());
    while (C.size() > 1 && C.back() == 0) C.pop_back();
    return C;
}