[LeetCode] Multiply Strings

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.

class Solution {
public:
    string multiply(string num1, string num2) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int size1=num1.length();
        int size2=num2.length();
        
        vector<int> resultInt;
        
        int carry=0;
        int d1=0,d2=0,mul=0;
        int i=0,j=0;
        for(i=0;i<size2;i++)
        {
            carry=0;
            d1=num2[size2-1-i]-'0';
            for(j=0;j<size1;j++)
            {
                d2=num1[size1-1-j]-'0';
                mul=d1*d2+carry;
                
                if(resultInt.size() < i+j+1)
                {
                    resultInt.push_back(mul%10);
                }
                else
                {
                    mul=resultInt[i+j]+mul;
                    resultInt[i+j]=mul%10;
                }
                carry=mul/10;
            }
            if(carry!=0)
            {
                if(resultInt.size()<i+j+2)
                    resultInt.push_back(carry);
                else
                    resultInt[i+j+1]=carry;
            }
        }
        
        //turn vector<int> to string arrays
        string result="";
        int allzero=1 
        for(int i=resultInt.size()-1;i>=0;i--)
        {
            if(resultInt[i]!=0)
                allzero=0;
                
            if(!allzero || i==0)
                result+='0'+resultInt[i];
        }
        return result;
    }
};