quick sort [random select pivot](https://suanfa8.com/quick-sort/random-select-pivot)

QuickSort.java

import java.util.Random;


class Solution {

    private final static Random random = new Random(System.currentTimeMillis());

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

    private void quickSort(int[] nums, int left, int right) {
        if (left >= right) {
            return;
        }

        int pivotIndex = partition(nums, left, right);
        quickSort(nums, left, pivotIndex - 1);
        quickSort(nums, pivotIndex + 1, right);
    }

    private int partition(int[] nums, int left, int right) {
        // [left..right]
        int randomIndex = left + random.nextInt(right - left + 1);
        swap(nums, left, randomIndex);

        int pivot = nums[left];
        
        // j记录pivot对应数值的最后位置+1
        int j = left + 1;
        // all in nums[left + 1..j) <= pivot
        // all in nums[j..i) > pivot
        for (int i = left + 1; i <= right; i++){
            // 把小于pivot的值交换到左面，j记录下一个待交换的位置
            if (nums[i] <= pivot) {
                swap(nums, i, j);
                j++;
            }
        }
        swap(nums, left, j - 1);
        return j - 1;
    }

    private void swap(int[] nums, int index1, int index2) {
        int temp = nums[index1];
        nums[index1] = nums[index2];
        nums[index2] = temp;
    }

}