题解 | 调整牛群的位置

1. 原地修改只能反来反去了。
2. 学一学 steam API

import java.util.*;
import java.util.stream.*;

public class Solution {
    private void reverse(final int[] a, int l, int r) {
        while (l < r) {
            final int tmp = a[l];
            a[l] = a[r];
            a[r] = tmp;
            ++l;
            --r;
        }
    }

    public ArrayList<Integer> rotate_left (int[] nums, int k) {
        final int n = nums.length;
        k = k % n;
        if (k != 0) {
            reverse(nums, 0, k - 1);
            reverse(nums, k, n - 1);
            reverse(nums, 0, n - 1);
        } 
        return new ArrayList<>(Arrays.stream(nums).boxed().collect(Collectors.toList()));
    }
}