如何使用Java更有效地反转数字

Question

I wrote some code to reverse a number as below:

        long num = 123456789;

        char[] arr = String.valueOf(num).toCharArray();
        List<Character> characterList = new ArrayList<Character>();
        for (char c : arr) {
            characterList.add(c);
        }
        Collections.reverse(characterList);

        for (Character c : characterList) {
            System.out.println(c);
        }

output:

9
8
7
6
5
4
3
2
1

Could anyone advise me a more efficient way to achieve this with Java?

EDIT :
In fact, the first guide about this question is to print them backwards, please just ignore this way.

Answer 1

Why use chars? You can do it directly using integer operations:

public static void main(String[] args) {
    long num = 123456789;

    while (num != 0) {
        System.out.println(num % 10);
        num = num / 10;
    }
}

Output:

9
8
7
6
5
4
3
2
1

This is pretty much what Long.toString() does internally , so it should be more efficient than working with the String.

Answer 2

You can go with only one loop:

long num = 123456789;

char[] arr = String.valueOf(num).toCharArray();
for(int i = arr.length - 1; i >= 0; i--) {
    System.out.println(arr[i]);
}

Answer 3

If by "more efficient" you mean fewer lines, you could use:

char[] reversed = new StringBuilder(String.valueOf(num))
                             .reverse().toString().toCharArray();

Answer 4

Simplest way:

long num = 123456789;

StringBuilder sb=new StringBuilder(num+"");

System.out.println(""+sb.reverse());