和大数相加差不多，需要注意两点：

• 先要注意判断，大数减小数正常相减，小数减大数相当于大数减小数前添加个负号
• 也是从后向前相减，个位开始相减，相减时先加个 10
• 最后注意去掉结果开头的 0

public class Huawei {

    public static void main(String[] args) {
        String num1 = "2000";
        String num2 = "1001";

        System.out.println(func(num1, num2));
    }

    private static String func(String num1, String num2) {
        char[] c1 = num1.toCharArray();
        char[] c2 = num2.toCharArray();

        int len1 = c1.length;
        int len2 = c2.length;

        if (len1 > len2) {
            return sub(c1, c2);
        }
        else if (len1 < len2) {
            return "-" + sub(c2, c1);
        }
        else {
            int compare = num1.compareTo(num2);
            if (compare == 0) {
                return "0";
            }
            else if (compare > 0) {
                return sub(c1, c2);
            }
            else {
                return "-" + sub(c2, c1);
            }
        }

    }

    private static String sub(char[] c1, char[] c2) {
        StringBuilder res = new StringBuilder();

        int len1 = c1.length;
        int len2 = c2.length;

        // 借位
        int carry = 0;

        for (int i = len1 - 1, j = len2 - 1; i >= 0 || j >= 0; i --, j --) {
            int a = (i >= 0 ? c1[i] - '0' : 0) + carry;
            int b = j >= 0 ? c2[j] - '0' : 0;

            int temp = 0;

            if (a < b) {
                // 借位
                carry = -1;
                temp = a + 10 - b;
            }
            else {
                carry = 0;
                temp = a - b;
            }

            res.append(temp);
        }
		
        // 去掉开头的 0
        if (res.charAt(res.length() - 1) == '0') {
            res.deleteCharAt(res.length() - 1);
        }

        return res.reverse().toString();
    }
}