Multiply Strings

class Solution {
public:
    string multiply(string num1, string num2) {
        int len1=num1.length(),len2=num2.length();
        string res(len1+len2,‘0‘);
        for(int i=len1-1;0<=i;i--){
            int carry=0;
            for(int j=len2-1;0<=j;j--){
                int tmp=(res[i+j+1]-‘0‘)+(num1[i]-‘0‘)*(num2[j]-‘0‘)+carry;
                res[i+j+1]=tmp%10+‘0‘;//各位处理
                carry=tmp/10;
            }
            res[i]+=carry;//最高位处理
        }
         size_t startpos = res.find_first_not_of("0");
         if (string::npos != startpos) {
            return res.substr(startpos);
        }
        return "0";    
    }
};