LeetCode题解之Multiply Strings

1、题目描述

2、问题分析

按照手算乘法的过程进行计算，首先计算乘法，然后计算加法。

3、代码

string multiply(string num1, string num2) {
        string small ;
        string big;
        if( num1.size() < num2.size() ){
            small = num1;
            big = num2;
        }else if( num2.size() < num1.size() ){
            small = num2;
            big = num1;
        }else{
            small = num1;
            big = num2;
        }
        
        int n = small.size() ;
        vector<string> re;
        
        string::reverse_iterator it_s = small.rbegin() ;
        while( it_s != small.rend() ){
            string  s;
            int up = 0;
            string::reverse_iterator it_b = big.rbegin() ;
            while( it_b != big.rend() ){
                int m = (*it_s - '0') * (*it_b - '0') + up;
                if(  m < 10 ){
                    s = std::to_string(m) + s;
                    up = 0;
                }else{
                    s = std::to_string( m%10 )  + s;
                    up = m/10;
                }
                ++it_b;
            }
            if( up != 0 ){
                s = std::to_string( up ) + s;
            }
            re.push_back( s );
            ++it_s;
        }
        
        string zero;
        for( vector<string>::iterator it = re.begin() ; it != re.end() ; it++ ){
            *it += zero;
            zero += "0";
        }
        
        string result = "0" ;
        for( int i = 0 ; i < re.size() ; i++ ){
            result = binaryAdd( result , re[i] );
        }
        
        int q = 0;
        for( string::iterator it = result.begin() ; it != result.end() ; ++it ){
            q += (*it - '0');
        }
        if( q == 0)
            result = "0";
        return result;
        
        
    }
    
    string binaryAdd( string s1 , string s2 ){
        string s;
        string::reverse_iterator it1 = s1.rbegin();
        string::reverse_iterator it2 = s2.rbegin();
        int up = 0;
        while( it1 != s1.rend() && it2 != s2.rend() ){
            int a = (*it1 - '0') + (*it2 - '0') + up;
            if( a < 10 ){
                s  = std::to_string(a) + s;
                up = 0;
            }else{
                s = std::to_string( a - 10 ) + s;
                up = 1;
            }
            ++it1 ;
            ++it2 ;
        }
        
        while( it1 != s1.rend() ){
            if( *it1 - '0' + up < 10 ){
                s = std::to_string( *it1 - '0' + up ) + s;
                up = 0;
            }else{
                s = std::to_string( *it1 - '0' + up - 10 ) + s;
                up = 1;
            }
            ++it1;
        }
        
        while( it2 != s2.rend() ){
            if( *it2 - '0' + up < 10 ){
                s = std::to_string( *it2 - '0' + up ) + s;
                up = 0;
            }else{
                s = std::to_string( *it2 - '0' + up -10  ) + s;
                up = 1;
            }
            ++it2;
        }
        
        if( up == 1){
            s = std::to_string(1) + s;
        }
        return s;
    }