[编程珠玑读书笔记]快速排序

一步一步优化的快速排序，速度比插入排序快很多，而且可以很方便的引入随机算法来避免最差情况。

#include<iostream>
#include<string.h>
#include<stdio.h>
#include<algorithm>
using namespace std;

const int MAX = 1000000;
void swap( int *data, int i, int j)
{
	int temp = data[i];
	data[i] = data[j];
	data[j] = temp;
}
void InsertSort3( int *data, int n)
{
	int i, j, temp;
	for( i = 1; i< n; i++)
	{
		temp = data[i];
		for( j = i; data[j - 1] > temp&& j > 0; j--)
			data[j ] = data[j - 1];
		data[j] = temp;	
	}
}

void QuickSort1( int *data, int l, int u)
{
	if( l >= u) return ;
	int m = l;
	for( int i = l + 1; i <=u; i++)
	{
		if( data[i] < data[l] )
			swap(data, ++m, i);
	}
	swap(data, l, m);
	QuickSort1(data, l, m - 1);
	QuickSort1(data, m + 1, u);
}

void QuickSort2( int *data, int l ,int u)
{
	int i, j;
	if ( l >= u) return;
	int temp;
	temp = data[l];
	i = l;
	j = u + 1;
	while( true)
	{
		do  i++;  while( i <= u && data[i] < temp);
		do  j--;  while( data[j] > temp);
		if( i > j )
			break;		
		swap( data, i, j);

	} 
	swap( data, l, j);
	QuickSort2(data, l , j -1);
	QuickSort2(data, j + 1 , u);
}

void QuickSort3( int *data, int l ,int u)
{
	int i, j;
	if ( u - l < 50) return;
	swap(data,l, rand() %( u - l ) + l);
	int temp;
	temp = data[l];
	i = l;
	j = u + 1;
	while( true)
	{
		do  i++;  while( i <= u && data[i] < temp);
		do  j--;  while( data[j] > temp);
		if( i > j )
			break;		
		int t = data[i]; data[i] = data[j]; data[j] = t;

	} 
	swap( data, l, j);
	QuickSort3(data, l , j -1);
	QuickSort3(data, j + 1 , u);
}
int main()
{
	srand(5);
	int data[ MAX];
	int data2[MAX];

	const int denominator = MAX * 10;
	double BegTime, EndTime;
	for( int i = 0; i < MAX; i++)
	{
		data[i] = rand() % denominator;
	}

	memcpy(data2, data, MAX);

	BegTime = clock();
	QuickSort1( data, 0, MAX - 1);
	EndTime = clock();
	printf("time of QucikSort1 test is : %g ms \n", (EndTime - BegTime ) / 1000);


	memcpy(data, data2, MAX);
	BegTime = clock();
	QuickSort2( data2, 0, MAX - 1);
	EndTime = clock();
	printf("time of QucikSort2 test is : %g ms \n", (EndTime - BegTime ) / 1000);

	BegTime = clock();
	QuickSort3( data, 0, MAX - 1);
	InsertSort3( data, MAX);
	EndTime = clock();
	printf("time of QucikSort3 && InsertSort test is : %g ms \n", (EndTime - BegTime ) / 1000);
	return 0;
}

执行结果如下所示：

time of QucikSort1 test is : 250 ms
time of QucikSort2 test is : 120 ms
time of QucikSort3 && InsertSort test is : 140 ms