# 快速排序

java

```package com.test.arithmetic;

import java.util.Arrays;

/**
* Two point go together, small is left and big is right.
* Thus the first meet will separate the array to half small and half big.
* Created by Ryan on 2017/3/25/025.
*/
public class QuickSort {
public  int count = 0;

public  int[] sort(int[] source, int low, int high) {
if (low >= high) {
return source;
}
int first = low;
int last = high;
int key = source[first];

while (first < last) {
//find the first smaller from the end
while (last > first && source[last] >= key) {
last--;
}
if (first < last) {
source[first] = source[last];
first++; //The first one has already been sorted, move to the next one.
}//else first = last, that is the same one, do not need to swap them

//find the first bigger from the start
while (first < last && source[first] <= key) {
first++;
}

if (first < last) {
source[last] = source[first];
last--; //The last one has already been sorted as a bigger, move to the previous one.
}

}

source[first] = key; //put the key
System.out.println("The " + ++count + " sort:");
Arrays.stream(source).forEach(item -> System.out.print(item + ", "));
System.out.println();

sort(source, low, first - 1);
sort(source, first + 1, high);
return source;

}

public void quickSort1(int arr[], int low, int high) {
int l = low;
int h = high;
int key = arr[low];

while (l < h) {
while (l < h && arr[h] >= key) {
h--;
}

if (l < h) {
int temp = arr[h];
arr[h] = arr[l];
arr[l] = temp;
l++;
}

while (l < h && arr[l] <= key) {
l++;
}

if (l < h) {
int temp = arr[h];
arr[h] = arr[l];
arr[l] = temp;
h--;
}
}
System.out.print("l=" + (l + 1) + "h=" + (h + 1) + "key=" + key + "\n");
if (l > low) quickSort1(arr, low, l - 1);
if (h < high) quickSort1(arr, l + 1, high);
}

public <T> void swap(T[] source, int i, int j) {
T temp = source[i];
source[i] = source[j];
source[j] = temp;
}

/**
* 方式2 更高效点的代码：
*/
public <T extends Comparable<? super T>> T[] quickSort2(T[] targetArr, int start, int end) {
int i = start + 1, j = end;
T key = targetArr[start];

if (start >= end) return (targetArr);

/**
* 从i++和j--两个方向搜索不满足条件的值并交换
*条件为：i++方向小于key，j--方向大于key
*/
while (true) {
while (targetArr[j].compareTo(key) > 0) j--;
while (targetArr[i].compareTo(key) < 0 && i < j) i++;
if (i >= j) break;
this.swap(targetArr, i, j);
if (targetArr[i] == key) {
j--;
} else {
i++;
}
}

/**
* 关键数据放到‘中间’*
*/
this.swap(targetArr, start, j);

if (start < i - 1) {
this.quickSort2(targetArr, start, i - 1);
}
if (j + 1 < end) {
this.quickSort2(targetArr, j + 1, end);
}

return targetArr;
}

/**
* 方式3：减少交换次数，提高效率
*/
public  <T extends Comparable<? super T>> void quickSort3(T[] targetArr, int start, int end) {
int i = start, j = end;
T key = targetArr[start];

while (i < j) {
/*按j--方向遍历目标数组，直到比key小的值为止*/
while (j > i && targetArr[j].compareTo(key) >= 0) {
j--;
}
if (i < j) {
/*targetArr[i]已经保存在key中，可将后面的数填入*/
targetArr[i] = targetArr[j];
i++;
}
/*按i++方向遍历目标数组，直到比key大的值为止*/
/*此处一定要小于等于零，假设数组之内有一亿个1，0交替出现的话，而key的值又恰巧是1的话，那么这个小于等于的作用就会使下面的if语句少执行一亿次。*/
while (i < j && targetArr[i].compareTo(key) <= 0) {
i++;
}
if (i < j) {
/*targetArr[j]已保存在targetArr[i]中，可将前面的值填入*/
targetArr[j] = targetArr[i];
j--;
}
/*此时i==j*/
targetArr[i] = key;

/*递归调用，把key前面的完成排序*/
this.quickSort3(targetArr, start, i - 1);

/*递归调用，把key后面的完成排序*/
this.quickSort3(targetArr, j + 1, end);
}
}

}```

Test:

```package com.test.arithmetic;

import org.junit.Assert;
import org.junit.Before;
import org.junit.Test;

import java.util.Arrays;

import static org.junit.Assert.*;

/**
* Created by Ryan on 2017/3/25/025.
*/
public class QuickSortTest {
private int[] source;
private Integer[] source2;
private Integer[] source3;
private QuickSort quick;

private int[] expect;
private Integer[] expect2;
private Integer[] expect3;

@Before
public void setUp(){
source = new int[]{13, 16, 1, 1, 5, 15, 13, 15, 13, 8};
source2 = new Integer[]{13, 16, 1, 1, 5, 15, 13, 15, 13, 8};
source3 = new Integer[]{13, 16, 1, 1, 5, 15, 13, 15, 13, 8};

expect = new int[]{1, 1, 5, 8, 13, 13, 13, 15, 15, 16};
expect2 = new Integer[]{1, 1, 5, 8, 13, 13, 13, 15, 15, 16};
expect3 = new Integer[]{1, 1, 5, 8, 13, 13, 13, 15, 15, 16};
quick = new QuickSort();
}

@Test
public void testSort() throws Exception {
int[] source = new int[]{13,16,1,1,5,15,13,15,13,8 };
int[] sort = quick.sort(source, 0, source.length - 1);
Assert.assertArrayEquals(expect, sort);
}
@Test
public void testSort1(){
QuickSort quick = new QuickSort();
quick.quickSort1(source, 0, source.length - 1);
Assert.assertArrayEquals(expect, source);
}

@Test
public void testSort2(){
QuickSort quick = new QuickSort();
quick.quickSort2(source2, 0, source2.length - 1);
Assert.assertArrayEquals(expect2, source2);
}
@Test
public void testSort3(){
QuickSort quick = new QuickSort();
quick.quickSort3(source3, 0, source.length - 1);
Assert.assertArrayEquals(expect3, source3);
}

}```

Result:

```l=5h=5key=13
l=4h=4key=8
l=1h=1key=1
l=3h=3key=5
l=2h=2key=1
l=9h=9key=15
l=6h=6key=13
l=7h=7key=13
l=8h=8key=15
l=10h=10key=16
The 1 sort:
8, 5, 1, 1, 13, 15, 13, 15, 13, 16,
The 2 sort:
1, 5, 1, 8, 13, 15, 13, 15, 13, 16,
The 3 sort:
1, 5, 1, 8, 13, 15, 13, 15, 13, 16,
The 4 sort:
1, 1, 5, 8, 13, 15, 13, 15, 13, 16,
The 5 sort:
1, 1, 5, 8, 13, 13, 13, 15, 15, 16,
The 6 sort:
1, 1, 5, 8, 13, 13, 13, 15, 15, 16,
The 7 sort:
1, 1, 5, 8, 13, 13, 13, 15, 15, 16, ```

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