题解 | #字符串加解密#利用对数组取模解决

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
       
        while (in.hasNextLine()) { 
            String a = in.nextLine();
            String b = in.nextLine();
            System.out.println(encrypt(a));
            System.out.println(decrypt(b));
        }
    }
    
    private static char[] mletter = "abcdefghijklmnopqrstuvwxyz".toCharArray();
    private static char[] cletter = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".toCharArray();
    private static char[] number = "0123456789".toCharArray();
    
    private static String encrypt(String input) {
        char[] arr = input.toCharArray();
        StringBuilder builder = new StringBuilder();
        for (int i = 0; i < arr.length; i++) {
            int index = (int)arr[i];
            if (arr[i] >= 'a' && arr[i] <= 'z') {
                builder.append(cletter[(index - 97 + 1) % 26]); // 字母a的int值是97,我们这里减成0后+1然后对26(a-z=26个字母)取模,再去大写数组里找对应下标就是了
            } else if (arr[i] >= 'A' && arr[i] <= 'Z') {
                builder.append(mletter[(index - 65 + 1) % 26]);
            } else if (arr[i] >= '0' && arr[i] <= '9') {
                builder.append(number[(index - 48 + 1) % 10]);
            } else {
                builder.append(arr[i]);
            }
        }
        return builder.toString();
    }
    
    private static String decrypt(String input) {
        char[] arr = input.toCharArray();
        StringBuilder builder = new StringBuilder();
        for (int i = 0; i < arr.length; i++) {
            int index = (int)arr[i];
            if (arr[i] >= 'a' && arr[i] <= 'z') {
                builder.append(cletter[(index - 97 + 26 - 1) % 26]); // 字母a的int值是97,为了防止负数出现,我们加26后再取模.PS:取模就跟拨时钟一样,12个钟多拨一圈会回到原点;)
            } else if (arr[i] >= 'A' && arr[i] <= 'Z') {
                builder.append(mletter[(index - 65 + 26 - 1) % 26]);
            } else if (arr[i] >= '0' && arr[i] <= '9') {
                builder.append(number[(index - 48 + 10 - 1) % 10]);
            } else {
                builder.append(arr[i]);
            }
        }
        return builder.toString();
    }
}