快排思想：

1. 查找一个数值pivot

2.在pivot两侧递归排序

3.实现pivot数值左侧比pivot小，右侧比pivot大。

代码实现：

package main

import "fmt"

func main() {
	nums := []int{50, 60, 70, 90, 80, 10, 20, 40}
	QuickSort(nums, 0, len(nums)-1)
	fmt.Println(nums)
}

func QuickSort(num []int, low int, high int) {
	if low < high {
		pos := Position(num, low, high)
		QuickSort(num, low, pos-1)
		QuickSort(num, pos+1, high)
	}
}

func Position(num []int, low int, high int) int {
	pivot := num[low]
	i := low
	j := high
	fmt.Println("i=", i, ";j=", j, ";pivot=", pivot)
	for i < j {
		for i < j && pivot <= num[j] {
			j--
		}
		num[i] = num[j]
		for i < j && pivot >= num[i] {
			i++
		}
		num[j] = num[i]
	}
	num[i] = pivot
	return i
}