大数加减乘模板

加法：

char * add(char *s, char *ss) {
    memset(str3, 0, sizeof(str3));
    int len1 = strlen(s)-1;
    int len2 = strlen(ss)-1;
    int len3 = 0, x, y, flag = 0;
    while(len1 >= 0 || len2 >= 0) {
        if(len1 >= 0) x = s[len1] - '0';
        else x = 0;
        if(len2 >= 0) y = ss[len2] - '0';
        else y = 0;
        str3[len3++] = (x+y+flag)%10 + '0';
        flag = (x+y+flag)/10;
        len2--;len1--;
    }
    if(flag) str3[len3++] = '1';
    for(int i = 0; i < len3/2; i ++) {
        swap(str3[i],str3[len3-i-1]);
    }
    return str3;
}

减法：

char *sub(char *s, char *ss) {
    memset(str3, 0, sizeof(str3));
    int len1 = strlen(s)-1, len2 = strlen(ss)-1;
    int flag = 1, len3 = 0, x, y, cnt = 0;
    if(strcmp(s,ss) == 0){
        str3[len3++] = '0';
        return str3;
    }
    if(len1 < len2 || (len1==len2&&strcmp(s,ss)<0)) {
        swap(len1,len2);
        swap(s,ss);
        flag = 0;
    }
    while(len1 >= 0 || len2 >= 0) {
        if(len1 >= 0) x = s[len1--] - '0';
        else x = 0;
        if(len2 >= 0) y = ss[len2--] - '0';
        else y = 0;
        int ans = x-y+cnt;
        if(ans < 0) {
            str3[len3++] = ans+10+'0';
            cnt = -1;
        } else {
            str3[len3++] = ans+'0';
            cnt = 0;
        }
    }
    while(str3[len3-1] == '0') {
        str3[--len3] = '';
    }
    if(!flag) str3[len3++] = '-';
    for(int i = 0; i < len3/2; i ++) swap(str3[i], str3[len3-i-1]);
    return str3;
}

乘法：

int mul(char *s, char *ss,int *num) {
    int len1 = strlen(s);
    int len2 = strlen(ss);
    int len3 = len1+len2;
    for(int i = 0; i < len1/2; i ++) swap(s[i],s[len1-i-1]);
    for(int i = 0; i < len2/2; i ++) swap(ss[i],ss[len2-i-1]);
    for(int i = 0; i < len1; i ++) {
        for(int j = 0; j < len2; j ++) {
            num[i+j] += (s[i]-'0')*(ss[j]-'0');
        }
    }
    for(int i = 0; i < len3; i ++) {
        if(num[i] >= 10) {
            num[i+1] += num[i]/10;
            num[i] %= 10;
        }
    }
    while(num[len3] == 0 && len3 >= 0) len3--;
    len3++;
    for(int i = 0; i < len3/2; i ++) {
        swap(num[i], num[len3-i-1]);
    }
    if(!len3) len3++;
    return len3;
}

std::string StringMul(const std::string& left, const std::string& right) { 
    bool flag1 = (left[0] == '-'), flag2 = (right[0] == '-');
    string s1, s2, result;
    if(flag1) s1 = left.substr(1,left.length());
    else s1 = left;
    if(flag2) s2 = right.substr(1,right.length());
    else s2 = right;
    int len1 = s1.length();
    int len2 = s2.length();
    int len3 = len1+len2;
    vector<int> num(len3+2, 0);
    for(int i = 0; i < len1/2; i ++) swap(s1[i],s1[len1-i-1]);
    for(int i = 0; i < len2/2; i ++) swap(s2[i],s2[len2-i-1]);
    for(int i = 0; i < len1; i ++) {
        for(int j = 0; j < len2; j ++) {
            num[i+j] += (s1[i]-'0')*(s2[j]-'0');
        }
    }
    for(int i = 0; i < len3; i ++) {
        if(num[i] >= 10) {
            num[i+1] += num[i]/10;
            num[i] %= 10;
        }
    }
    
    while(num[len3] == 0 && len3 >= 0) len3--;
    len3++;
    for(int i = 0; i < len3/2; i ++) {
        swap(num[i], num[len3-i-1]);
    }
    for(int i = 0; i < len3; i ++) {
        result = result + char(num[i]+'0');
    }
    if(!len3) return "0";
    if((flag1&&!flag2) || (!flag1&&flag2)) result = '-' + result;
    return result;
}

Java的加减乘法：

import java.util.Scanner;
import java.math.BigDecimal;
import java.math.BigInteger;

public class Main{
    public static void main(String[] args) {
        Scanner cin = new Scanner(System.in);
        BigDecimal a = cin.nextBigDecimal();
        BigDecimal b = cin.nextBigDecimal();
        BigInteger aa = cin.nextBigInteger();
        BigInteger bb = cin.nextBigInteger();
        System.out.println("a+b:"+a.add(b));
        System.out.println("a-b:"+a.subtract(b));
        System.out.println("a*b:"+a.multiply(b));
        System.out.println("a/b:"+aa.divide(bb));
        System.out.println("a^10:"+a.pow(10));
    }
}