字符串模拟大数相加

题目描述：
用字符串模拟两个大数相加。

    public static String bigNumberAdd(String str1, String str2) {

        //翻转
        char[] arr1 = new StringBuilder(str1).reverse().toString().toCharArray();
        char[] arr2 = new StringBuilder(str2).reverse().toString().toCharArray();

        int len1 = arr1.length;
        int len2 = arr2.length;

        int len = len1 > len2 ? len1 : len2;
        int[] result = new int[len + 1];

        for(int i = 0; i < len + 1; i++) {
            int num1 = i < len1 ? (arr1[i] - '0') : 0;
            int num2 = i < len2 ? (arr2[i] - '0') : 0;
            result[i] = num1 + num2;
        }

        //处理进位
        for(int i = 0; i < result.length; i++) {
            if(result[i] >= 10) {
                result[i+1] = result[i+1] + result[i] / 10;
                result[i] = result[i] % 10;
            }
        }

        StringBuilder sb = new StringBuilder();
        boolean flag = true;

        for(int i = len; i >= 0; i--) {
            if(result[i] == 0 && flag) {
                continue;
            } else {
                flag = false;
            }

            sb.append(result[i]);
        }

        return sb.toString();

    }