HackerRank 上的 Java 子字符串比较中的测试用例 2 失败

问：

它通过了除测试用例 2 和 4 之外的所有情况。这是我的代码：

import java.util.Scanner;

public class Solution {

    public static String getSmallestAndLargest(String s, int k) {
        String smallest = "";
        String largest = "";
        
        // Complete the function
        // 'smallest' must be the lexicographically smallest substring of length 'k'
        // 'largest' must be the lexicographically largest substring of length 'k'
        int min = Integer.MAX_VALUE;
        int max = Integer.MIN_VALUE;
        int len = s.length() - k;
        for (int i = 0; i <= len; i++) {
            String output = s.substring(i, k++);
            int ascii_code_1 = output.charAt(0);
            int ascii_code_2 = output.charAt(0);
            if (ascii_code_1 < min) {
                min = ascii_code_1;
                smallest = output;
            }
            if (ascii_code_2 > max) {
                max = ascii_code_2;
                largest = output;
            }
        }
        
        return smallest + "\n" + largest;
    }


    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        String s = scan.next();
        int k = scan.nextInt();
        scan.close();
      
        System.out.println(getSmallestAndLargest(s, k));
    }
}

下面是测试用例 2：

ASDFHDSFHsdlfhsdlfLDFHSDLFHsdlfhsdlhkfsdlfLHDFLSDKFHsdfhsdlkfhsdlfhsLFDLSFHSDLFHsdkfhsdkfhsdkfhsdfhsdfjeaDFHSDLFHDFlajfsdlfhsdlfhDSLFHSDLFHdlfhs
30