Java 代码

import java.io.*;
import java.util.*;

public class Main {

    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));

        String a = reader.readLine();
        String b = reader.readLine();

        List<Integer> diff = null;
        boolean negative = false;
        if (compare(a, b)) {
            diff = substract(a, b);
        } else {
            negative = true;
            diff = substract(b, a);
        }

        StringBuilder builder = new StringBuilder();

        for (int i = diff.size() - 1; i >= 0; i--) {
            if (diff.get(i) == 0 && builder.length() == 0) continue;
            builder.append(diff.get(i));
        }

        if (builder.length() == 0) {
            System.out.print(0);
        } else if (negative) {
            System.out.print('-');
            System.out.print(builder.toString());
        } else {
            System.out.print(builder.toString());
        }

    }

    public static boolean compare(String a, String b) {
        if (a.length() != b.length()) return a.length() > b.length();
        for (int i = 0; i < a.length(); i++) {
            if (a.charAt(i) != b.charAt(i)) {
                return a.charAt(i) > b.charAt(i);
            }
        }
        return true;
    }

    public static List<Integer> substract(String a, String b) {
        // a >= b
        List<Integer> diff = new ArrayList<>();
        int i = a.length() - 1;
        int j = b.length() - 1;

        int t = 0;
        while (i >= 0) {
            t = a.charAt(i--) - '0' - t;
            t -= j >= 0 ? b.charAt(j--) - '0' : 0;

            diff.add((t + 10) % 10);
            t = t < 0 ? 1 : 0;
        }
        return diff;
    }
}