大整数减法——C++实现

#include <iostream>
#include <string>
#include <algorithm>
using namespace std;

//将s1置为较大数
bool exchangeS(string &s1,string &s2,int &size1,int &size2){
    bool isExchange = false;
    if(size1 < size2){
        isExchange = true;
    }else if(size1 == size2){
        for(int i = 0;i != size1;i++){
            if(s1[i] < s2[i]){
                isExchange = true;
                break;  
            }
        }
    }

    if(isExchange){
        swap(size1,size2);
        swap(s1,s2);
    }
    return isExchange;
}

//大整数加法
string bigIntAdd(string &s1,string &s2){
    int size1 = s1.size();
    int size2 = s2.size();
    
    bool isExchange = exchangeS(s1,s2,size1,size2);
    
    bool isCarry = false;
    for(int i = 0;i != size1;i++){
        int addRes = s1[size1 - i - 1] - '0';
        if(i < size2){
            addRes += s2[size2 - i - 1] - '0';
        }else if(!isCarry){
            break;  
        }
        if(isCarry)
            addRes++;
        isCarry = addRes >= 10;
        addRes %= 10;
        s1[size1 - i - 1] = '0' + addRes;
    }
    
    if(isCarry){
        s1 = "1" + s1;      
    }
               
    return s1;             
}

//大整数减法
string bigIntSub(string s1,string s2){
    //s1为正，s2为负
    //s1为负，s2为正
    //采用大整数加法。
    bool isNegative1 = s1[0] == '-';
    bool isNegative2 = s2[0] == '-';
    if(isNegative1 && !isNegative2){
        s1.erase(s1.begin());
        bigIntAdd(s1,s2);
        return "-" + s1;
    }else if(isNegative2 && !isNegative1){
        s2.erase(s2.begin());
        bigIntAdd(s1,s2);
        return s1;
    }
    
    if(isNegative1 && isNegative2){
        s1.erase(s1.begin());
        s2.erase(s2.begin());
    }
    
    int size1 = s1.size();
    int size2 = s2.size();
    
    bool isExchange = exchangeS(s1,s2,size1,size2);
    
    bool isBorrow = false;
    for(int i = 0;i != size1;i++){
        char ch1 = s1[size1 - i - 1];
        
        if(i < size2){
            char ch2 = s2[size2 - i - 1];
            isBorrow = ch1 < ch2;
            int v = ch1 - ch2;
            if(isBorrow)
                v += 10;
            s1[size1 - i - 1] = '0' + v;            
        }else if(!isBorrow)
            break;
        else{
            if(isBorrow && s1[size1 - i - 1] > '0'){
                s1[size1 - i - 1] -= 1;
                if((size1 - i - 1) == 0 && s1[size1 - i - 1] == '0'){
                    s1.erase(s1.begin());   
                }
                break;
            }else{
                s1[size1 - i - 1] = '9';
            }
        }
    }
    if(isNegative1 && isNegative2)
        isExchange = !isExchange;
    if(isExchange)
        return "-" + s1;
    return s1;
}

int main()
{
    cout << bigIntSub("1111","222222") << endl;
    cout << bigIntSub("-1111","222222") << endl;
    cout << bigIntSub("-1111","-222222") << endl;
    cout << bigIntSub("1111","-222222") << endl;
    
   cout << "Hello World";
   return 0;
}