bigNumberAdd

void addNum(string& orgStr, string addStr) {
    string biggerStr, smallerStr;
    int frontNum=0;

    if (orgStr.size() >= addStr.size()) {
        biggerStr = orgStr;
        smallerStr = addStr;
    }
    else {
        biggerStr = addStr;
        smallerStr = orgStr;
    }//end of define biggerStr and smallerStr

    for (int i = 0; i < smallerStr.size(); i++) {
        int bigIndex = biggerStr.size() - i - 1;
        int smaIndex = smallerStr.size() - i - 1;

        biggerStr[bigIndex] += smallerStr[smaIndex] - '0';
    }//end of biggerStr add

    for (int i = 0; i < biggerStr.size(); i++) {
        int bigIndex = biggerStr.size() - i - 1;

        while (biggerStr[bigIndex] > '9') {
            if (bigIndex == 0) {
                frontNum++;
                biggerStr[bigIndex] -= 10;
            }//case of index is zero
            else {
                biggerStr[bigIndex] -= 10;
                biggerStr[bigIndex - 1] += 1;
            }//case of index is not zero

        }//end of while in for
    }//end of sorting str

    if (frontNum != 0) {
        string tempStr = "0";
        tempStr[0] = '0' + frontNum;
        biggerStr = tempStr+biggerStr;
    }


    //exchanege of orgStr;
    orgStr = biggerStr;
}//total time complex is n