examples: add quick_sort example

examples/quick_sort.v

@@ -0,0 +1,50 @@
+import time
+import rand
+
+const (
+	LEN = 1000 // how many random numbers to generate
+	MAX = 10000 // max of the generated numbers
+)
+
+fn main() {
+	rand.seed(time.now().unix)
+	rand.next(MAX) // skip the first
+	mut arr := []int
+	for _ in 0..LEN {
+		arr << rand.next(MAX)
+	}
+	println('length of random array is $arr.len')
+	println('before quick sort whether array is sorted: ${is_sorted(arr)}')
+	quick_sort(mut arr, 0, arr.len-1)
+	println('after quick sort whether array is sorted: ${is_sorted(arr)}')
+}
+
+fn quick_sort(arr mut []int, l int, r int) {
+	if l>=r { return }
+	mut sep := l  // what is sep: [...all_value<arr[sep]...sep...all_value>=arr[sep]...]
+	for i in l+1..r+1 {
+		if arr[i] < arr[l] {
+			sep++
+			swap(mut arr, i, sep)
+		}
+	}
+	swap(mut arr, l, sep)
+	quick_sort(mut arr, l, sep-1)
+	quick_sort(mut arr, sep+1, r)
+}
+
+[inline] 
+fn swap(arr mut []int, i int, j int) {
+	temp := arr[i]
+	arr[i] = arr[j]
+	arr[j] = temp
+}
+
+fn is_sorted(arr []int) bool {
+	for i in 0..arr.len-1 {
+		if arr[i] > arr[i+1] {
+			return false
+		}
+	}
+	return true
+}