LeetCode 43 Multiply Strings

class Solution {
public:
    int ans[10005];
    string multiply(string num1, string num2) {
        
        if(num1=="0"||num2=="0")
            return "0";
       
        int pos=0;
        int start=0;
        int len=0;
        memset(ans,0,sizeof(ans));
        for(int i=num2.size()-1;i>=0;i--)
        {
            int x = num2[i]-'0';
          
            pos=start;
            for(int j=num1.size()-1;j>=0;j--)
            {
                int y = x*(num1[j]-'0');
                ans[pos] += y;
                
                int tag=pos;
                while(ans[tag]>=10)
                {
                    ans[tag+1]+=ans[tag]/10;
                    ans[tag]=ans[tag]%10;
                    tag++;
                    len=max(len,tag+1);
                }
                pos++;
                len=max(len,pos);
            }
            start++;
        }
        string res;
        for(int i=len-1;i>=0;i--)
        {
            
            res+=ans[i]+'0'; 
        }
        return res;
        
    }
};