对冒泡排序的一种优化（减少外层循环次数）

 今天碰巧看到了冒泡培训的舞蹈视频，不过在看的过程中受到一点启发，才有了这次优化。网上有很多对冒泡排序的优化，这里提供一种不同的方案，已有的不再讨论。冒泡排序效率问题，估计很少使用，这里只作为学习讨论之用。

 

优化方案：我们都知道，对于长度为n的数组来说，冒泡排序外层循环需要n-1趟排序。然而如果当某趟排序的最后两个元素不需要调换位置时，我们可以减少一次外层循环。例如：设j+1为此趟排序的最后一个元素，如果当arr[j]<=arr[j+1]时，则在之后的排序中，不需要其他元素再与arr[j]进行比较，即减少一次外层循环。

 

package test;
public class BubbleSort {
   public static void main(String[] args) {

      int[] arr = {5,0,1,6,2,7,3,8,4,9};

      System.out.println("初始顺序为：");

      for(int i:arr){

         System.out.print(i + " ");

      }
      System.out.println();

      int[] result = bubbleSort(arr);

      System.out.println("排序后结果为：");

      for(int i:result){

         System.out.print(i + " ");

      }

     

   }

  

   public static int[] bubbleSort(int[] arr){

      int[] src = arr.clone();

      int times = 0;

      for(int i = 0; i < src.length -1; i++){

         System.out.println("-------------------------------");

         System.out.println("第" + (++times) + "次排序");

         for(int j = 0; j < src.length - 1 - i;j++){

            System.out.println("compare src["+j+"]="+src[j]+"andsrc["+(j+1)+ "]=" + src[j+1]);

            if(src[j] > src[j + 1]){

                int tmp = src[j];

                src[j] = src[j + 1];

                src[j + 1] = tmp;

            }

            /**

             * 优化：

             * 如果此趟排序中最后两个元素不需要交换位置，则减少一趟排序

             */

            else if(j == src.length - 1 - i - 1){

                i++;

            }

         }

        

         // 输出此趟排序结果

         for(int num:src){

            System.out.print(num + " ");

         }

         System.out.println();

      }

     

      System.out.println("共经历["+times+"]次排序");

      return src;

   }

  

}

 

输入结果：

初始顺序为：

5 0 1 6 2 7 3 8 4 9

-------------------------------

第0次排序

comepare src[0]=5 and src[1]=0

comepare src[1]=5 and src[2]=1

comepare src[2]=5 and src[3]=6

comepare src[3]=6 and src[4]=2

comepare src[4]=6 and src[5]=7

comepare src[5]=7 and src[6]=3

comepare src[6]=7 and src[7]=8

comepare src[7]=8 and src[8]=4

comepare src[8]=8 and src[9]=9

0 1 5 2 6 3 7 4 8 9

-------------------------------

第2次排序

comepare src[0]=0 and src[1]=1

comepare src[1]=1 and src[2]=5

comepare src[2]=5 and src[3]=2

comepare src[3]=5 and src[4]=6

comepare src[4]=6 and src[5]=3

comepare src[5]=6 and src[6]=7

comepare src[6]=7 and src[7]=4

0 1 2 5 3 6 4 7 8 9

-------------------------------

第3次排序

comepare src[0]=0 and src[1]=1

comepare src[1]=1 and src[2]=2

comepare src[2]=2 and src[3]=5

comepare src[3]=5 and src[4]=3

comepare src[4]=5 and src[5]=6

comepare src[5]=6 and src[6]=4

0 1 2 3 5 4 6 7 8 9

-------------------------------

第4次排序

comepare src[0]=0 and src[1]=1

comepare src[1]=1 and src[2]=2

comepare src[2]=2 and src[3]=3

comepare src[3]=3 and src[4]=5

comepare src[4]=5 and src[5]=4

0 1 2 3 4 5 6 7 8 9

-------------------------------

第5次排序

comepare src[0]=0 and src[1]=1

comepare src[1]=1 and src[2]=2

comepare src[2]=2 and src[3]=3

comepare src[3]=3 and src[4]=4

0 1 2 3 4 5 6 7 8 9

-------------------------------

第7次排序

comepare src[0]=0 and src[1]=1

comepare src[1]=1 and src[2]=2

0 1 2 3 4 5 6 7 8 9

共经历[6]次排序

排序后结果为：

0 1 2 3 4 5 6 7 8 9

 

 

为了对比，下面是优化之前的输出：

初始顺序为：

5 0 1 6 2 7 3 8 4 9

---------------------------------------------------

第0次排序

comepare src[0]=5 and src[1]=0

comepare src[1]=5 and src[2]=1

comepare src[2]=5 and src[3]=6

comepare src[3]=6 and src[4]=2

comepare src[4]=6 and src[5]=7

comepare src[5]=7 and src[6]=3

comepare src[6]=7 and src[7]=8

comepare src[7]=8 and src[8]=4

comepare src[8]=8 and src[9]=9

0 1 5 2 6 3 7 4 8 9

---------------------------------------------------

第1次排序

comepare src[0]=0 and src[1]=1

comepare src[1]=1 and src[2]=5

comepare src[2]=5 and src[3]=2

comepare src[3]=5 and src[4]=6

comepare src[4]=6 and src[5]=3

comepare src[5]=6 and src[6]=7

comepare src[6]=7 and src[7]=4

comepare src[7]=7 and src[8]=8

0 1 2 5 3 6 4 7 8 9

---------------------------------------------------

第2次排序

comepare src[0]=0 and src[1]=1

comepare src[1]=1 and src[2]=2

comepare src[2]=2 and src[3]=5

comepare src[3]=5 and src[4]=3

comepare src[4]=5 and src[5]=6

comepare src[5]=6 and src[6]=4

comepare src[6]=6 and src[7]=7

0 1 2 3 5 4 6 7 8 9

---------------------------------------------------

第3次排序

comepare src[0]=0 and src[1]=1

comepare src[1]=1 and src[2]=2

comepare src[2]=2 and src[3]=3

comepare src[3]=3 and src[4]=5

comepare src[4]=5 and src[5]=4

comepare src[5]=5 and src[6]=6

0 1 2 3 4 5 6 7 8 9

---------------------------------------------------

第4次排序

comepare src[0]=0 and src[1]=1

comepare src[1]=1 and src[2]=2

comepare src[2]=2 and src[3]=3

comepare src[3]=3 and src[4]=4

comepare src[4]=4 and src[5]=5

0 1 2 3 4 5 6 7 8 9

---------------------------------------------------

第5次排序

comepare src[0]=0 and src[1]=1

comepare src[1]=1 and src[2]=2

comepare src[2]=2 and src[3]=3

comepare src[3]=3 and src[4]=4

0 1 2 3 4 5 6 7 8 9

---------------------------------------------------

第6次排序

comepare src[0]=0 and src[1]=1

comepare src[1]=1 and src[2]=2

comepare src[2]=2 and src[3]=3

0 1 2 3 4 5 6 7 8 9

---------------------------------------------------

第7次排序

comepare src[0]=0 and src[1]=1

comepare src[1]=1 and src[2]=2

0 1 2 3 4 5 6 7 8 9

---------------------------------------------------

第8次排序

comepare src[0]=0 and src[1]=1

0 1 2 3 4 5 6 7 8 9

共经历[9]次排序

排序后结果为：

0 1 2 3 4 5 6 7 8 9