Rotating an int Array in Java using only one semicolon

问题

I'm doing a bunch of voluntaty tasks for Java, which cover working with basic arrays and methods. I've been doing quite well, until I hit a severe roadblock.

The initial task was to create a method rotateByN, which should take an int array and rotate it by N steps (and affect the input array, not return a new one). Simple enough:

public class Four {
    static void rotateByN(int[] in, int n) {
        for (int l = 1; l <= n; l++) {
            int tmp = in[0];
            for (int i = 1; i < in.length; i++) {
                in[i - 1] = in[i];
            }
            in[in.length - 1] = tmp;
        }
    }

    public static void main(String[] args) {
        int[] arr = { 2, 7, 6, 1, 0 };
        rotateByN(arr, 3);
        for (int r : arr) {
            System.out.println(r);
        }
    }
}

However, the follow up task was to reduce the amounts of semicolons used in the method body to only one. I've spent about an hour crawling through documentation of all sorts of possibilities, but nothing seemed to get me there.

The more frustrating part is that it wasn't clarified if we could just call a previously created function rotate (which does the same thing, but always exactly once) N times, but then again, a for loop would need more semicolons which we don't have.

I hate to be asking for something so bluntly, but I'm completely out of options, and any kind of input would be greatly appreciated.

After some more questioning, it turns out to be as boring as calling the rotate method over and over again. Since this can only (I think) be done with more semicolons in the for head, perhaps they aren't counted.

回答1:

You can use Arrays.stream(int[],int,int) method two times to get two streams with the specified ranges of array: near and far, then swap and flatten them to one stream - return new array. Then you can use System.arraycopy method to replace the contents of the old array with the contents of the new one. Long path with one semicolon:

static void rotateByN(int[] arr, int n) {
    System.arraycopy(Stream
            .of(Arrays.stream(arr, n, arr.length),
                    Arrays.stream(arr, 0, n))
            .flatMapToInt(Function.identity())
            .toArray(), 0, arr, 0, arr.length);
}

public static void main(String[] args) {
    int[] arr = {2, 7, 6, 1, 0};
    rotateByN(arr, 3);
    System.out.println(Arrays.toString(arr)); // [1, 0, 2, 7, 6]
}

Or you can use Arrays.copyOfRange(int[],int,int) method. The idea is the same:

static void rotateByN(int[] arr, int n) {
    System.arraycopy(Stream
            .of(Arrays.copyOfRange(arr, n, arr.length),
                    Arrays.copyOfRange(arr, 0, n))
            .flatMapToInt(Arrays::stream)
            .toArray(), 0, arr, 0, arr.length);
}

Or without streams you can use ArrayUtils.addAll(int[],int[]) method:

static void rotateByN(int[] arr, int n) {
    System.arraycopy(ArrayUtils.addAll(
            Arrays.copyOfRange(arr, n, arr.length),
            Arrays.copyOfRange(arr, 0, n)),
            0, arr, 0, arr.length);
}

来源：https://stackoverflow.com/questions/65060947/rotating-an-int-array-in-java-using-only-one-semicolon

标签

java

arrays

rotation