java - Swap the adjacent character only if string has even length else print invalid length

Question

import java.io.*;
import java.util.*;
class Main {
    static String solve(String str) throws Exception {
        return "";
    }

    public static void main(String []args) throws Exception {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        int t = Integer.parseInt(br.readLine());

        for (int i = 0; i < t; i++) {
            String str = br.readLine();
            try {
                System.out.println(solve(str));
            } catch(Exception e) {
                System.out.println(e.getMessage());
            }
        }
    }
}

The first line of input contains a single integer representing the number of string to be inputted such as 2 so string will be:

hgtsgf
tadad

If the string has even characters without any numeric value then swap the adjacent and if string contains odd characters then print invalid string as output.

question from:https://stackoverflow.com/questions/65844064/swap-the-adjacent-character-only-if-string-has-even-length-else-print-invalid-le

Answer 1

Taking into account the following statement:

If the string has even characters without any numeric value then swap the adjacent and if string contains odd [number of] characters then print invalid string as output

The solution may be as follows:

public static String solve(String str) {
    String result = "Invalid string";  // prepare result
    if (null != str && str.length() % 2 == 0) { // check the null and length of the input
        
        char[] arr = str.toCharArray();
        
        for (int i = 0; i < arr.length; i += 2) {
            if (Character.isDigit(arr[i]) || Character.isDigit(arr[i + 1])) {
                // if any digit found, return "Invalid string"
                return result;
            }
            // do the swap
            char t = arr[i];
            arr[i] = arr[i + 1];
            arr[i + 1] = t;
        }
        // make the result string
        result = new String(arr);
    }
    return result;
}

Tests:

String[] tests = {
    "abcd", "AB", "", "AAaaBBbb", // even length
    "1234", // digits
    "AbCdE", "A", // odd length
    null,   
    "A.B+C-D!" // even length without digits
};
for (String t : tests) {
    System.out.printf("%s -> %s%n", t, solve(t));
}

Output:

abcd -> badc
AB -> BA
 -> 
AAaaBBbb -> AAaaBBbb
1234 -> Invalid string
AbCdE -> Invalid string
A -> Invalid string
null -> Invalid string
A.B+C-D! -> .A+B-C!D