题目描述：

Given two non-negative integers num1 and num2 represented as string, return the sum of num1 and num2.

Note:

1. The length of both num1 and num2 is < 5100.
2. Both num1 and num2 contains only digits 0-9.
3. Both num1 and num2 does not contain any leading zero.
4. You must not use any built-in BigInteger library or convert the inputs to integer directly.

public String addStrings(String num1, String num2) {
        int len = Math.max(num1.length(), num2.length());
        char ch[] = new char[len + 1];
        int i = num1.length() - 1, j = num2.length() - 1;
        int c = 0, k = len;
        while(i >= 0 && j >= 0){
            int tempI = num1.charAt(i) - '0';
            int tempJ = num2.charAt(j) - '0';
            int tempSum = tempI + tempJ + c;
            ch[k] = String.valueOf(tempSum % 10).charAt(0);
            c = tempSum / 10;
            --k;
            --i;
            --j;
        }
        while(i >= 0) {
            int tempI = num1.charAt(i) - '0';
            int tempSum = tempI + c;
            ch[k] = String.valueOf(tempSum % 10).charAt(0);
            c = tempSum / 10;
            --k;
            --i;
        }
        while(j >= 0) {
            int tempJ = num2.charAt(j) - '0';
            int tempSum = tempJ + c;
            ch[k] = String.valueOf(tempSum % 10).charAt(0);
            c = tempSum / 10;
            --k;
            --j;
        }
        int flag = 0;
        if(c != 0) {
            flag = 1;
            ch[0] = String.valueOf(c).charAt(0);
        }
        String tempStr = String.valueOf(ch);
        if(flag == 0)
            return tempStr.substring(1);
        else
            return tempStr;
    }