排序算法的Java实现全攻略

Collections.sort()

Java的排序可以用Collections.sort() 排序函数实现。
用Collections.sort方法对list排序有两种方法：
第一种是list中的对象实现Comparable接口，如下：

/**
* 根据order对User排序
*/
public class User implements Comparable<User>{
private String name;
private Integer order;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Integer getOrder() {
return order;
}
public void setOrder(Integer order) {
this.order = order;
}
public int compareTo(User arg0) {
return this.getOrder().compareTo(arg0.getOrder());
}
}

测试一下：

public class Test{
public static void main(String[] args) {
User user1 = new User();
user1.setName("a");
user1.setOrder(1);
User user2 = new User();
user2.setName("b");
user2.setOrder(2);
List<User> list = new ArrayList<User>();
//此处add user2再add user1
list.add(user2);
list.add(user1);
Collections.sort(list);
for(User u : list){
System.out.println(u.getName());
}
}
}

输出结果如下

a
b

第二种方法是根据Collections.sort重载方法来实现，例如：

/**
* 根据order对User排序
*/
public class User { //此处无需实现Comparable接口
private String name;
private Integer order;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Integer getOrder() {
return order;
}
public void setOrder(Integer order) {
this.order = order;
}
}

主类中这样写即可：

public class Test{
public static void main(String[] args) {
User user1 = new User();
user1.setName("a");
user1.setOrder(1);
User user2 = new User();
user2.setName("b");
user2.setOrder(2);
List<User> list = new ArrayList<User>();
list.add(user2);
list.add(user1);
Collections.sort(list,new Comparator<User>(){
public int compare(User arg0, User arg1) {
return arg0.getOrder().compareTo(arg1.getOrder());
}
});
for(User u : list){
System.out.println(u.getName());
}
}
}

输出结果如下

a
b

前者代码结构简单，但是只能根据固定的属性排序，后者灵活，可以临时指定排序项，但是代码不够简洁

择优用之。

常用排序算法
下面来看几种经典排序算法的Java代码实践：

冒泡排序

public static void bubbleSort(int A[], int n) {
int i, j;
for (i = 0; i < n - 1; i ++) {
for (j = 0; j < n - i - 1; j ++) {
if (A[j] > A[j + 1]) {
A[j] = A[j] ^ A[j + 1];
A[j + 1] = A[j] ^ A[j + 1];
A[j] = A[j] ^ A[j + 1];
}
}
}
}

直接插入排序

public static void insertSort(int A[], int n) {
int i, j, tmp;
for (i = 1; i < n; i++) {
tmp = A[i];
for (j = i - 1; j >= 0; j--) {
if (A[j] > tmp) {
A[j + 1] = A[j];
} else {
break;
}
}
A[j + 1] = tmp;
}
}

直接选择排序

public static void selectSort(int A[], int n) {
int i, j, loc;
for (i = 0; i < n; i++) {
loc = i;
for (j = i + 1; j < n; j++) {
if (A[j] < A[loc]) {
loc = j;
}
}
if (loc != i) {
A[i] = A[i] ^ A[loc];
A[loc] = A[i] ^ A[loc];
A[i] = A[i] ^ A[loc];
}
}
}

堆排序

/**
* 堆排序（从小到大）
*
* @param A
* @param n
*/
public static void heapSort(int A[], int n) {
int tmp;
// 构建大根堆
buildMaxHeap(A, n);
for (int j = n - 1; j >= 1; j--) {
tmp = A[0];
A[0] = A[j];
A[j] = tmp;
maxheapIfy(A, 0, j);
}
}
/**
* 构建大根堆
*
* @param A
* @param n
*/
private static void buildMaxHeap(int A[], int n) {
for (int i = (n - 2) / 2; i >= 0; i--) {
maxheapIfy(A, i, n);
}
}
/**
* 维护从下标i开始的最大堆
*
* @param A
* @param i
* @param n
*/
private static void maxheapIfy(int A[], int i, int n) {
int left, right, loc;
while (i < n) {
left = 2 * i + 1;
right = 2 * i + 2;
loc = i;
if (left < n && A[left] > A[i]) {
i = left;
}
if (right < n && A[right] > A[i]) {
i = right;
}
if (loc != i) {
A[i] = A[loc] ^ A[i];
A[loc] = A[loc] ^ A[i];
A[i] = A[loc] ^ A[i];
} else {
break;
}
}
}

快速排序

public static void quickSort(int A[], int bt, int ed) {
if (bt < ed) {
int pivot = pivotPartition(A, bt, ed);
quickSort(A, bt, pivot - 1);
quickSort(A, pivot + 1, ed);
}
}
private static void swapVar(int A[], int bt, int ed) {
int mid = bt + (ed - bt) / 2;
if (mid != bt) {
A[bt] = A[bt] ^ A[mid];
A[mid] = A[bt] ^ A[mid];
A[bt] = A[bt] ^ A[mid];
}
}
private static int pivotPartition(int A[], int bt, int ed) {
// 取中间值作为stand，防止数组有序出现O(n^2)情况
swapVar(A, bt, ed);
int stand = A[bt];
while (bt < ed) {
while (bt < ed && A[ed] >= stand) {
ed--;
}
if (bt < ed) {
A[bt++] = A[ed];
}
while (bt < ed && A[bt] <= stand) {
bt++;
}
if (bt < ed) {
A[ed--] = A[bt];
}
}
A[bt] = stand;
return bt;
}

归并排序

public static void mergeSort(int A[], int bt, int ed) {
if (bt < ed) {
int mid = bt + (ed - bt) / 2;
mergeSort(A, bt, mid);
mergeSort(A, mid + 1, ed);
mergeArray(A, bt, mid, ed);
}
}
private static void mergeArray(int A[], int bt, int mid, int ed) {
int i, j, k, len = ed - bt + 1;
int tmp[] = new int[len];
for (i = bt, j = mid + 1, k = 0; i <= mid && j <= ed; k++) {
if (A[i] <= A[j]) {
tmp[k] = A[i++];
} else {
tmp[k] = A[j++];
}
}
while (i <= mid) {
tmp[k++] = A[i++];
}
while (j <= ed) {
tmp[k++] = A[j++];
}
for (i = 0; i < k; i++) {
A[bt + i] = tmp[i];
}
}

测试程序

 来将以上算法归纳总结一下：

import java.util.Scanner;
public class JavaSort {
public static void main(String args[]) {
Scanner cin = new Scanner(System.in);
int A[], n;
while (cin.hasNext()) {
n = cin.nextInt();
A = new int
;
for (int i = 0; i < n; i++) {
A[i] = cin.nextInt();
}
// bubbleSort(A, n);
// insertSort(A, n);
// selectSort(A, n);
// heapSort(A, n);
// quickSort(A, 0, n - 1);
mergeSort(A, 0, n - 1);
printArr(A);
}
}
/**
* 归并排序
*
* @param A
* @param bt
* @param ed
*/
public static void mergeSort(int A[], int bt, int ed) {
if (bt < ed) {
int mid = bt + (ed - bt) / 2;
mergeSort(A, bt, mid);
mergeSort(A, mid + 1, ed);
mergeArray(A, bt, mid, ed);
}
}
/**
* 合并数组
*
* @param A
* @param bt
* @param mid
* @param ed
*/
private static void mergeArray(int A[], int bt, int mid, int ed) {
int i, j, k, len = ed - bt + 1;
int tmp[] = new int[len];
for (i = bt, j = mid + 1, k = 0; i <= mid && j <= ed; k++) {
if (A[i] <= A[j]) {
tmp[k] = A[i++];
} else {
tmp[k] = A[j++];
}
}
while (i <= mid) {
tmp[k++] = A[i++];
}
while (j <= ed) {
tmp[k++] = A[j++];
}
for (i = 0; i < k; i++) {
A[bt + i] = tmp[i];
}
}
/**
* 快速排序
*
* @param A
* @param bt
* @param ed
*/
public static void quickSort(int A[], int bt, int ed) {
if (bt < ed) {
int pivot = pivotPartition(A, bt, ed);
quickSort(A, bt, pivot - 1);
quickSort(A, pivot + 1, ed);
}
}
private static void swapVar(int A[], int bt, int ed) {
int mid = bt + (ed - bt) / 2;
if (mid != bt) {
A[bt] = A[bt] ^ A[mid];
A[mid] = A[bt] ^ A[mid];
A[bt] = A[bt] ^ A[mid];
}
}
/**
* 快排寻找基准点位置
*
* @param A
* @param bt
* @param ed
* @return
*/
private static int pivotPartition(int A[], int bt, int ed) {
// 取中间值作为stand，防止数组有序出现O(n^2)情况
swapVar(A, bt, ed);
int stand = A[bt];
while (bt < ed) {
while (bt < ed && A[ed] >= stand) {
ed--;
}
if (bt < ed) {
A[bt++] = A[ed];
}
while (bt < ed && A[bt] <= stand) {
bt++;
}
if (bt < ed) {
A[ed--] = A[bt];
}
}
A[bt] = stand;
return bt;
}/**
* 堆排序（从小到大）
*
* @param A
* @param n
*/
public static void heapSort(int A[], int n) {
int tmp;
// 构建大根堆
buildMaxHeap(A, n);
for (int j = n - 1; j >= 1; j--) {
tmp = A[0];
A[0] = A[j];
A[j] = tmp;
maxheapIfy(A, 0, j);
}
}
/**
* 构建大根堆
*
* @param A
* @param n
*/
private static void buildMaxHeap(int A[], int n) {
for (int i = (n - 2) / 2; i >= 0; i--) {
maxheapIfy(A, i, n);
}
}
/**
* 维护从下标i开始的最大堆
*
* @param A
* @param i
* @param n
*/
private static void maxheapIfy(int A[], int i, int n) {
int left, right, loc;
while (i < n) {
left = 2 * i + 1;
right = 2 * i + 2;
loc = i;
if (left < n && A[left] > A[i]) {
i = left;
}
if (right < n && A[right] > A[i]) {
i = right;
}
if (loc != i) {
A[i] = A[loc] ^ A[i];
A[loc] = A[loc] ^ A[i];
A[i] = A[loc] ^ A[i];
} else {
break;
}
}
}/**
* 直接选择排序
*
* @param A
* @param n
*/public static void selectSort(int A[], int n) {
int i, j, loc;
for (i = 0; i < n; i++) {
loc = i;
for (j = i + 1; j < n; j++) {
if (A[j] < A[loc]) {
loc = j;
}
}
if (loc != i) {
A[i] = A[i] ^ A[loc];
A[loc] = A[i] ^ A[loc];
A[i] = A[i] ^ A[loc];
}
}
}/**
* 直接插入排序
*
* @param A
* @param n
*/public static void insertSort(int A[], int n) {
int i, j, tmp;
for (i = 1; i < n; i++) {
tmp = A[i];
for (j = i - 1; j >= 0; j--) {
if (A[j] > tmp) {
A[j + 1] = A[j];
} else {
break;
}
}
A[j + 1] = tmp;
}
}/**
* 冒泡排序
*
* @param A
* @param n
*/
public static void bubbleSort(int A[], int n) {
int i, j;
for (i = 0; i < n - 1; i++) {
for (j = 0; j < n - i - 1; j++) {
if (A[j] > A[j + 1]) {
A[j] = A[j] ^ A[j + 1];
A[j + 1] = A[j] ^ A[j + 1];
A[j] = A[j] ^ A[j + 1];
}
}
}
}
/**
* 打印数组
*
* @param A
*/
public static void printArr(int A[]) {
for (int i = 0; i < A.length; i++) {
if (i == A.length - 1) {
System.out.printf("%d\n", A[i]);
} else {
System.out.printf("%d ", A[i]);
}
}
}
}

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