415 字符串相加

思路：每个字符串一个指针
tips:注意res=最新的结果+res，不用最后再反转

class Solution {
public:
    string addStrings(string num1, string num2) {

        //大数相加
        int l1=num1.size(),l2=num2.size();
        int p1=l1-1,p2=l2-1;
        int carry=0;
        string res="";
       
        int i=0;
        while(p1>=0 || p2>=0 || carry!=0)
        {
            if(p1<0 && p2<0)
            {
                res=to_string(carry)+res;
                carry=0;
                break;
            }
            else if(p1<0)
            {
                while(p2>=0)
                {
                    res=to_string((num2[p2]-'0'+carry)%10)+res;
                    carry=(num2[p2]-'0'+carry)/10;
                    i++;
                    p2--;
                }
            }
            else if(p2<0)
            {
                 while(p1>=0)
                {
                    res=to_string((num1[p1]-'0'+carry)%10)+res;
                    carry=(num1[p1]-'0'+carry)/10;
                    i++;
                    p1--;
                }

            }
            else
            {
                
                res=to_string((num1[p1]-'0'+num2[p2]-'0'+carry)%10)+res;
                carry=(num1[p1]-'0'+num2[p2]-'0'+carry)/10;
                p1--;
                p2--;
                i++;

            }
        }



        return res;



    }
};