快速排序及其优化

快速排序及其优化

public class QuickSort {public static void main(String[] args) {int [] a ={1,2,3,0,9,8,7,6,5,4};Sort(a,10);for(int k=0;k<a.length ;k++){System.out.print(a[k]+"   ");}}private static void Sort(int[] k,int n) {QSort(k,0,n-1);}private static void QSort(int [] k,int low,int high) {int point;if(low<high){point =Partition(k,low,high);QSort(k,low,point-1);QSort(k,point+1,high);}}private static int Partition(int[] k,int low,int high) {int point;point=k[low];while(low<high){while(low<high&&k[high]>=point){high--;}swap(k,low,high);while(low<high&&k[low]<=point){low++;}swap(k,low,high);}return low;}private static void swap(int[] k, int low, int high) {int temp;temp=k[low];k[low]=k[high];k[high]=temp;}}

优化一：优化选取基准点

取出数组中的头部，中间，尾部三个元素并比较大小，然后将三个值中最大的放到k[high],最小值放到k[middle],将中间值放到k[low]，并设为point点。

private static int Partition(int[] k,int low,int high) {int point;int m=low+(high-low)/2;if(k[low]>k[high]){swap(k,low,high);}if(k[m]>k[high]){swap(k,m,high);}if(k[m]>k[low]){swap(k,m,low);}point=k[low];while(low<high){while(low<high&&k[high]>=point){high--;}swap(k,low,high);while(low<high&&k[low]<=point){low++;}swap(k,low,high);}return low;}

优化二：优化不必要的交换

这里有很多交换操作都是可以避免的。

private static int Partition(int[] k,int low,int high) {int point;int m=low+(high-low)/2;if(k[low]>k[high]){swap(k,low,high);}if(k[m]>k[high]){swap(k,m,high);}if(k[m]>k[low]){swap(k,m,low);}point=k[low];while(low<high){while(low<high&&k[high]>=point){high--;}//swap(k,low,high);k[low]=k[high];while(low<high&&k[low]<=point){low++;}//swap(k,low,high);k[high]=k[low];}k[low]=point;return low;}

优化三：优化小数组时的排序方案

在处理小于7个数的排序的时候，不必使用快速排序的方式了，这样效率会降低，使用直接插入排序的方式更优化。

private static void QSort(int [] k,int low,int high) {int point;if(high-low>7){point =Partition(k,low,high);QSort(k,low,point-1);QSort(k,point+1,high);}else{InsertSort(k,low,high);}}private static void InsertSort(int[] k, int low, int high) {int temp,j;for(int i=low;i<high;i++){if(k[i+1]<k[i]){temp=k[i+1];for( j=i;j>=low&&k[j]>temp;j--){k[j+1]=k[j];}k[j+1]=temp;}}}

如果是c语言的话还有优化四--递归的优化，利用尾调用可以进一步的让快速排序的速度更快。但是java不支持尾调用，这里就不详细介绍了。

下面是三个优化完成后的完整代码：

public class QuickSortImprove {public static void main(String[] args) {int [] a ={1,2,3,0,9,8,7,6,5,4};Sort(a,10);for(int k=0;k<a.length ;k++){System.out.print(a[k]+"   ");}}private static void Sort(int[] k,int n) {QSort(k,0,n-1);}private static void QSort(int [] k,int low,int high) {int point;if(high-low>7){point =Partition(k,low,high);QSort(k,low,point-1);QSort(k,point+1,high);}else{InsertSort(k,low,high);}}private static void InsertSort(int[] k, int low, int high) {int temp,j;for(int i=low;i<high;i++){if(k[i+1]<k[i]){temp=k[i+1];for( j=i;j>=low&&k[j]>temp;j--){k[j+1]=k[j];}k[j+1]=temp;}}}private static int Partition(int[] k,int low,int high) {int point;int m=low+(high-low)/2;if(k[low]>k[high]){swap(k,low,high);}if(k[m]>k[high]){swap(k,m,high);}if(k[m]>k[low]){swap(k,m,low);}point=k[low];while(low<high){while(low<high&&k[high]>=point){high--;}k[low]=k[high];while(low<high&&k[low]<=point){low++;}k[high]=k[low];}k[low]=point;return low;}private static void swap(int[] k, int low, int high) {int temp;temp=k[low];k[low]=k[high];k[high]=temp;}}