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:
    void Plus(string& a,string& b,string& temp){
        int small=min(a.length(),b.length());
        bool flag=false;
        int i=0;
        for(;i<small;i++){
            temp[i]=a[i]+b[i]-'0';
            if(flag)temp[i]++;
            if(temp[i]>'9'){
                temp[i]-=10;
                flag=true;
            }
            else{
                flag=false;
            }
        }
        while(i<a.length()){
            temp[i]=a[i];
            if(flag)temp[i]++;
            if(temp[i]>'9'){
                temp[i]-=10;
                flag=true;
            }
            else{
                flag=false;
            }
            i++;
        }
        while(i<b.length()){
            temp[i]=b[i];
            if(flag)temp[i]++;
            if(temp[i]>'9'){
                temp[i]-=10;
                flag=true;
            }
            else{
                flag=false;
            }
            i++;
        }
        if(flag){
            temp[i]='1';
            i++;
        }
        a.resize(i);
        for(int j=0;j<i;j++)a[j]=temp[j];
    }
    void MultiplyByNum(string& a,int val,int tens,string&temp){
        int j=0;
        for(;j<tens;j++){
            temp[j]='0';
        }
        int f=0;
        for(int i=0;i<a.length();i++,j++){
            int c=(a[i]-'0')*val;
            c+=f;
            temp[j]=c%10+'0';
            f=c/10;
        }
        while(f!=0){
            temp[j]=f%10+'0';
            f=f/10;
            j++;
        }
        a.resize(j);
        for(int i=0;i<j;i++)a[i]=temp[i];
    }
    string multiply(string num1, string num2) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        string temp;
        if(num1.length()==0||num2.length()==0)return "0";
        temp.resize(num1.length()+num2.length()+1);
        for(int i=0;i<num1.size()/2;i++){
            char c=num1[i];
            num1[i]=num1[num1.size()-1-i];
            num1[num1.size()-1-i]=c;
        }
        for(int i=0;i<num2.size()/2;i++){
            char c=num2[i];
            num2[i]=num2[num2.size()-1-i];
            num2[num2.size()-1-i]=c;
        }
        string p;
        string sum;
        for(int i=0;i<num2.length();i++){
            int val=num2[i]-'0';
            if(val!=0){
                p=num1;
                MultiplyByNum(p,val,i,temp);
                Plus(sum,p,temp);
            }
        }
        if(sum=="")return "0";
        bool allzero=true;
        for(int i=0;i<sum.length();i++){
            if(sum[i]!='0'){
                allzero=false;
                break;
            }
        }
        if(allzero)return "0";
        for(int i=0;i<sum.size()/2;i++){
            char c=sum[i];
            sum[i]=sum[sum.size()-1-i];
            sum[sum.size()-1-i]=c;
        }
        
        return sum;
    }
};