快速排序

class Solution {
    public int[] sortArray(int[] nums) {
        quicksort(nums, 0, nums.length - 1);
        return nums;
    }

    public void quicksort(int[] nums, int le, int ri) {
        if (le < ri) {
            int pos = randomizedPartition(nums, le, ri);
            quicksort(nums, le, pos - 1);
            quicksort(nums, pos + 1, ri);
        }
    }

    public int randomizedPartition(int[] nums, int le, int ri) {
        int i = new Random().nextInt(ri - le + 1) + le; // 随机选一个作为我们的主元
        exch(nums, ri, i);
        return partition(nums, le, ri);
    }

    public int partition(int[] nums, int le, int ri) {
        int pivot = nums[ri];
        int i = le - 1;
        for (int j = le; j <= ri - 1; ++j) {
            if (nums[j] <= pivot) {
                i = i + 1;
                exch(nums, i, j);
            }
        }
        exch(nums, i + 1, ri);
        return i + 1;
    }

    private void exch(int[] nums, int i, int j) {
        int temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;
    }
}