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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[n]; 
   
        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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