public static String addStrings(String num1, String num2) {

        int ptrNum1 = num1.length()-1 ;

        int ptrNum2 = num2.length()-1 ;

        int count = 0;

        StringBuilder result = new StringBuilder();

        //1、从两数个位开始相加

        while(ptrNum1>=0&&ptrNum2>=0){

            //[a、换算成数字]

            int a = num1.charAt(ptrNum1)-'0';

            int b = num2.charAt(ptrNum2)-'0';

            //[b、计算位数之和，包含进位count]

            int sum = a+b+count;

            count = 0;

            //[c、如果和大于9，则有新进位]

            if(sum>9){

                count =1;

                sum-=10;

            }

            //[d、将处理后sum插入结果]

            result.insert(0,sum);

            ptrNum1--;

            ptrNum2--;

        }

        //2、假如数字1还有高位未相加

        while(ptrNum1>=0){

            //[a、仅仅与进位值相加]

            int a =num1.charAt(ptrNum1)-'0'+count;

            count = 0;

            if(a>=10){

                count =1;

                a-=10;

            }

            //[b、将处理后sum插入结果]

            result.insert(0,a);

            ptrNum1--;

        }

        //3、加入数字2还有高位未相加

        while(ptrNum2>=0){

            int a =num2.charAt(ptrNum2)-'0'+count;

            count = 0;

            if(a>10){

                count =1;

                a-=10;

            }

            result.insert(0,a);

            ptrNum2--;

        }

        //4、考虑最高位进位的情况

        if(count==1)

            result.append("1");

        return result.toString();

    }