高精度加法（可以為負數）【C++版和java版】

高精度加法：

兩個整數相加：

#include <cstdio>
#include <cstring>
using namespace std;
const int MAX=1001;
void big_add(char a[],char b[],char c[])
{
    int i,e=0,d;
    int len1=strlen(a);
    int len2=strlen(b);
    for(i=0;i<len1/2;i++)
    {
        char t=a[i];a[i]=a[len1-1-i];a[len1-1-i]=t;
    }
    for(i=0;i<len2/2;i++)
    {
        char t=b[i];b[i]=b[len2-1-i];b[len2-1-i]=t;
    }
    for(i=0;i<len1&&i<len2;i++)
    {
        d=a[i]-'0'+b[i]-'0'+e;
        c[i]=d%10+'0';
        e=d/10;
    }
    if(i<len1)
      for(;i<len1;i++)
      {
          d=a[i]-'0'+e;
          c[i]=d%10+'0';
          e=d/10;
      }
    else
      for(;i<len2;i++)
      {
          d=b[i]-'0'+e;
          c[i]=d%10+'0';
          e=d/10;
      }
    if(e) c[i++]=e+'0';
    c[i]=0;
    int len3=i;
    for(i=0;i<len3/2;i++)
    {
        char t=c[i];c[i]=c[len3-1-i];c[len3-1-i]=t;
    }
}
int main()
{
    char a[MAX],b[MAX],c[MAX];
    while(scanf("%s%s",a,b)!=-1)
    {
        big_add(a,b,c);
        printf("%s\n",c);
    }
    return 0;
}

不限制正負：

#include<stdio.h>
#include<stdlib.h>
#include<string.h>

char A[10005];
char B[10005];
int fa, fb;
void swap(char *a, int i, int j)
{
	char t = a[i]-'0';
	a[i] = a[j]-'0';
	a[j] = t;
}
void add(char *a, char *b)
{
	int la = strlen(a),lb = strlen(b);
	int i,j,c=0,s,l;
	for(i=fa,j=la-1; i<=j; ++i,--j) swap(a,i,j);
	for(i=fb,j=lb-1; i<=j; ++i,--j) swap(b,i,j);
	for(i=fa; i<la||i<lb; ++i)
	{   
		s = a[i] + b[i] + c;
		c = s/10;
		a[i] = s%10;
	}
	a[i] = c;
	l = c ? i : i-1;
	if(fa) printf("-");
	for(i=l; i>=fa; --i) printf("%d", a[i]);
}

int cmp(char *a, char *b)
{
	int i,j,la,lb;
	la = strlen(a);
	lb = strlen(b);
	if(la-fa>lb-fb)
		return 1;
	else if(la-fa<lb-fb)
		    return 0;
	else
	{
		for(i=0; i<la&&a[i+fa]==b[i+fb]; ++i);
		return a[i+fa]>b[i+fb];
	}	
}
void minus(char *a, char *b)
{
	char *t;
	int i,j,ft,la,lb,c,l,s;
	if(!cmp(a,b))
	{
		t=a; a = b; b = t;
		ft = fa; fa = fb; fb = ft;
	}
	la = strlen(a);
	lb = strlen(b);
	for(i=fa,j=la-1; i<=j; ++i,--j) swap(a,i,j);
	for(i=fb,j=lb-1; i<=j; ++i,--j) swap(b,i,j);
	c = 0;
	l = -1;
	for(i=0; i+fa<la; ++i)
	{
		s = a[i+fa]-b[i+fb]-c>=0 ? 0 : 1;
		a[i+fa] = (10+a[i+fa]-b[i+fb]-c)%10;
		l = a[i+fa] ? i+fa : l;
		c = s;
	}
	if(l<0)
		printf("0");
	else
	{
		if(fa) printf("-");
	    for(i=l; i>=fa; --i) printf("%d", a[i]);
	}
}
int main()
{
	scanf("%s%s", A, B);
	fa = ('-'==A[0]);
	fb = ('-'==B[0]);
	if(fa^fb)
		minus(A,B);
	else
		add(A,B);
}

java版：

import java.util.*;
import java.io.*;
import java.lang.String;
import java.math.BigDecimal;

public class p1036
{
    public static void main(String[] args)
    {
        String s1,s2;
        Scanner cin = new Scanner(System.in);        
        s1 = cin.next();
        s2 = cin.next();
        BigDecimal b1 = new BigDecimal(s1);
        BigDecimal b2 = new BigDecimal(s2);
        System.out.println(b1.add(b2));
        cin.close();
    }
}