记录一个当时没有做出来的超大数相加

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主要是 一个 正数 一个负数的情况。
保证测试用例 输入字符串是正常的正负整数。
正负数的字符串长度 不超过 10^9.

public class LeetCode {
    public static void main(String[] args) {
        System.out.println(new LeetCode().addString("-12312", "12312"));
        System.out.println(-12312 + 12312);

        System.out.println(new LeetCode().addString("-145", "12312"));
        System.out.println(-145 + 12312);

        System.out.println(new LeetCode().addString("154545", "954545"));
        System.out.println(154545 + 954545);

        System.out.println(new LeetCode().addString("123", "5689"));
        System.out.println(123 + 5689);

        System.out.println(new LeetCode().addString("-123", "-5689"));


    }

    public String addString(String a, String b) {
        if ((a.charAt(0) == '-') != (b.charAt(0) == '-')) {
            // 一正一负
            if (a.charAt(0) == '-') {
                return check(a, b);
            } else {
                return check(b, a);
            }
        } else if (a.charAt(0) == '-' && b.charAt(0) == '-') {
            //都是负数
            return "-" + addString(a.substring(1), b.substring(1));
        }
        //都是正数
        char[] arr1 = a.toCharArray();
        char[] arr2 = b.toCharArray();
        int len = Math.max(arr1.length, arr2.length) + 1;
        int[] res = new int[len];
        int t = 0;
        for (int i = len - 1; i >= 0; i--) {
            int indexA = (arr1.length - (len - i));
            int indexB = (arr2.length - (len - i));
            int numA = indexA < 0 ? 0 : (arr1[indexA] - '0');
            int numB = indexB < 0 ? 0 : (arr2[indexB] - '0');
            t += numA + numB;
            res[i] = t % 10;
            t = t / 10;
        }
        int i = 0;
        while (res[i] == 0) i++;
        StringBuilder builder = new StringBuilder();
        while (i < len) builder.append(res[i++]);
        return builder.toString();
    }

    private String check(String negative, String positive) {
        if (negative.length() - 1 > positive.length()) {
            return "-" + sub(negative.substring(1), positive);
        } else if (negative.length() - 1 < positive.length()) {
            return sub(positive, negative.substring(1));
        } else {
            for (int i = 0; i < positive.length(); i++) {
                if (negative.charAt(i + 1) > positive.charAt(i)) {
                    return "-" + sub(negative.substring(1), positive);
                } else if (negative.charAt(i + 1) < positive.charAt(i)) {
                    return sub(positive, negative.substring(1));
                }
            }
            return "0";
        }
    }


    private String sub(String a, String b) {
        //a > b 求 a - b
        char[] arrA = a.toCharArray();
        char[] arrB = b.toCharArray();
        int t = 0;
        for (int i = 0; i < arrA.length; i++) {
            int index = arrB.length - 1 - i;
            int numB = index < 0 ? 0 : (arrB[arrB.length - 1 - i] - '0');
            int numA = arrA[arrA.length - 1 - i] - '0';
            t = numA - numB - t;
            if (t >= 0) {
                arrA[arrA.length - 1 - i] = (char) ('0' + t);
                t = 0;
            } else {
                arrA[arrA.length - 1 - i] = (char) ('0' + (10 + t));
                t = 1;
            }
        }
        return new String(arrA);
    }
}

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