***(leetcode_string) Multiply Strings

Multiply Strings

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Given two numbers represented as strings, return multiplication of the numbers as a string.

Note: The numbers can be arbitrarily large and are non-negative.

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class Solution { 
    string multiplySingle(string str, char ch, int num){
        if(ch=='0') return "0";
        int j = str.length()-1;
        string ans(num,'0');
        int b=ch-'0', carry = 0;
        for(;j>=0;j--){
           ans=((char)(((str[j]-'0')*b+carry)%10+'0')+ans);
           carry=((str[j]-'0')*b+carry)/10;
        }
        if(carry!=0)
            return ((char)(carry+'0'))+ans;
        else
            return ans;
    }
    string sum(string str1, string str2){
        int i = str1.length()-1;
        int j = str2.length()-1;
        int carry = 0;
        string ans="";
        while(i>=0&&j>=0){
            ans=((char)(((str1[i]-'0')+(str2[j]-'0')+carry)%10+'0'))+ans;
            carry=((str1[i]-'0')+(str2[j]-'0')+carry)/10;
            i--,j--;
        }
        while(i>=0){
            ans=((char)(((str1[i]-'0')+carry)%10+'0'))+ans;
            carry=((str1[i]-'0')+carry)/10;
            i--;
        }
        while(j>=0){
            ans=((char)(((str2[j]-'0')+carry)%10+'0'))+ans;
            carry=((str2[j]-'0')+carry)/10;
            j--;
        }
        if(carry!=0)
            return ((char)(carry+'0'))+ans;
        else
            return ans;
    }
public:
    string multiply(string num1, string num2) {
        stack<string> sumStack;
        int i,j;
        for(i=num2.length()-1,j=0 ; i>=0 ; i--,j++){
            sumStack.push(multiplySingle(num1, num2[i],j));
        }
        while(sumStack.size()!=1){
            string s1 = sumStack.top();
            sumStack.pop();
            string s2 = sumStack.top();
            sumStack.pop();
            sumStack.push(sum(s1,s2));
        }
        string tmp = sumStack.top();
        if(tmp[0]!='0') return tmp;
        for(i=0;i<tmp.length();i++)
            if(tmp[i]!='0')
                return tmp.substr(i,tmp.length()-i-1);
        return "0";
    }
    
};