快速排序优化，二路快排，三路快排

Author: tianqing.liang

08-QuickSort-More/ArrayGenerator.dart

@@ -0,0 +1,21 @@
+import 'dart:math';
+
+class ArrayGenerator{
+
+  static List generateOrderedArray(int n){
+     List arr =  List.filled(n, true);
+     for(int i = 0;i<n;i++){
+       arr[i] =i;
+     }
+     return arr;
+  }
+
+  static List generateRandomArray(int n,int bound){
+    List arr = List.filled(n, true);
+    Random random = new Random();
+    for(int i = 0; i < n; i ++){
+      arr[i] = random.nextInt(bound);
+    }
+    return arr;
+  }
+}

08-QuickSort-More/QuickSort2way.dart

@@ -0,0 +1,59 @@
+import 'ArrayGenerator.dart';
+import 'dart:math';
+
+/**
+ * 二路快排
+ */
+class QuickSort2way {
+  _QuickSort2way() {}
+
+  static sort2ways(List arr) {
+    Random rnd = new Random();
+    _sort2waysDetail(arr, 0, arr.length - 1, rnd);
+  }
+
+  static _sort2waysDetail(List arr, int l, int r, Random rnd) {
+    if (l >= r) return;
+
+    int p = _partition2ways(arr, l, r, rnd);
+    _sort2waysDetail(arr, l, p - 1, rnd);
+    _sort2waysDetail(arr, p + 1, r, rnd);
+  }
+
+  static int _partition2ways(List arr, int l, int r, Random rnd) {
+    int p = l + rnd.nextInt(r - l + 1);
+    _swap(arr, l, p);
+    // arr[l+1...i-1] <= v; arr[j+1...r] >= v
+    int i = l + 1, j = r;
+    while (true) {
+      while (i <= j && arr[i].compareTo(arr[l]) < 0){
+        i++;
+      }
+      while (j >= i && arr[j].compareTo(arr[l]) > 0){
+        j--;
+      }
+      if (i >= j){
+        break;
+      }
+      _swap(arr, i, j);
+      i++;
+      j--;
+    }
+    _swap(arr, l, j);
+    return j;
+  }
+
+  static _swap(List arr, int i, int j) {
+    var t = arr[i];
+    arr[i] = arr[j];
+    arr[j] = t;
+  }
+}
+
+void main() {
+  int n = 100;
+  List arr2 = ArrayGenerator.generateRandomArray(n, n);
+  print(arr2);
+  QuickSort2way.sort2ways(arr2);
+  print(arr2);
+}

08-QuickSort-More/QuickSort3way.dart

@@ -0,0 +1,78 @@
+import 'ArrayGenerator.dart';
+import 'dart:math';
+
+/**
+ * 二路快排
+ */
+class QuickSort3way {
+  _QuickSort3way() {}
+
+  static sort3ways(List arr) {
+    Random rnd = new Random();
+    _sort3waysDetail(arr, 0, arr.length - 1, rnd);
+  }
+
+  static _sort3waysDetail(List arr, int left, int right, Random rnd) {
+    if (left >= right) return;
+    // 生成 [l, r] 之间的随机索引
+    int p = left + rnd.nextInt(right - left + 1);
+    _swap(arr, left, p);
+    // arr[l + 1, lt] < v, arr[lt + 1, i - 1] == v, arr[gt, r] > v
+    int lt = left, i = left + 1, gt = right + 1;
+    while(i < gt){
+      if(arr[i].compareTo(arr[left]) < 0){
+        lt ++;
+        _swap(arr, i, lt);
+        i ++;
+      }
+      else if(arr[i].compareTo(arr[left]) > 0){
+        gt --;
+        _swap(arr, i, gt);
+      }
+      else{ // arr[i] == v
+        i ++;
+      }
+    }
+    _swap(arr, left, lt);
+    // 递归调用
+    _sort3waysDetail(arr, left, lt - 1, rnd);
+    _sort3waysDetail(arr, gt, right, rnd);
+  }
+
+  static int _partition3ways(List arr, int l, int r, Random rnd) {
+    int p = l + rnd.nextInt(r - l + 1);
+    _swap(arr, l, p);
+    // arr[l+1...i-1] <= v; arr[j+1...r] >= v
+    int i = l + 1, j = r;
+    while (true) {
+      while (i <= j && arr[i].compareTo(arr[l]) < 0){
+        i++;
+      }
+      while (j >= i && arr[j].compareTo(arr[l]) > 0){
+        j--;
+      }
+      if (i >= j){
+        break;
+      }
+      _swap(arr, i, j);
+      i++;
+      j--;
+    }
+    _swap(arr, l, j);
+    return j;
+  }
+
+  static _swap(List arr, int i, int j) {
+    var t = arr[i];
+    arr[i] = arr[j];
+    arr[j] = t;
+  }
+}
+
+void main() {
+  int n = 1000;
+  List arr2 = ArrayGenerator.generateRandomArray(n, n);
+  print(arr2);
+  QuickSort3way.sort3ways(arr2);
+  print(arr2);
+}

08-QuickSort/QuickSort.dart

@@ -1,4 +1,5 @@
 import 'ArrayGenerator.dart';
+import 'dart:math';
 class QuickSort{
   _QuickSort(){
   }
@@ -16,6 +17,9 @@ class QuickSort{
   }
 
   static int _partition(List arr,int left,int right){
+    //优化中间值选取，采用随机方式
+    int p = left+(new Random().nextInt(right-left+1));
+    _swap(arr, left, p);
     int j =left;
     for(var i = left+1;i<=right;i++){
         if(arr[i].compareTo(arr[left])<0){

README.md

@@ -9,7 +9,7 @@
 5. 链表，链表实现栈，链表实现队列
 6. 递归
 7. 归并排序,归并排序排序优化,自底向上排序
-8. 快速排序
+8. 快速排序,快速排序优化，二路快排，三路快排
 
 #### SDK版本
 1. 版本：2.12.3