JAVA排序算法之 选择排序

1. 选择排序

选择排序的基本思想是遍历数组的过程中，以i代表当前需要排序的序号，则需要在剩余的[i..n-1]中找出其中的最小值，然后将找到的最小值与i指向的值进行交换。因为每一次确定元素的过程中都会有一个选择很大值的子流程，所以称之为选择排序。

比如[38, 17, 16, 16, 7, 31, 39, 32, 2, 11]

i = 0: [2 , 17, 16, 16, 7, 31, 39, 32, 38 , 11] (0th [38]<->8th [2])

i = 1: [2, 7 , 16, 16, 17 , 31, 39, 32, 38, 11] (1st [38]<->4th [17])

i = 2: [2, 7, 11 , 16, 17, 31, 39, 32, 38, 16 ] (2nd [11]<->9th [16])

i = 3: [2, 7, 11, 16, 17, 31, 39, 32, 38, 16] ( 无需交换 )

i = 4: [2, 7, 11, 16, 16 , 31, 39, 32, 38, 17 ] (4th [17]<->9th [16])

i = 5: [2, 7, 11, 16, 16, 17 , 39, 32, 38, 31 ] (5th [31]<->9th [17])

i = 6: [2, 7, 11, 16, 16, 17, 31 , 32, 38, 39 ] (6th [39]<->9th [31])

i = 7: [2, 7, 11, 16, 16, 17, 31, 32, 38, 39] ( 无需交换 )

i = 8: [2, 7, 11, 16, 16, 17, 31, 32, 38, 39] ( 无需交换 )

i = 9: [2, 7, 11, 16, 16, 17, 31, 32, 38, 39] ( 无需交换 )

选择排序随着排序的进行(i逐渐增大)，比较的次数会越来越少，但是不论数组初始是否有序，选择排序都会从i至数组末尾进行一次选择比较，所以给定长度的数组，选择排序的比较次数是固定的:1+2+3+…+n=n*(n+1)/2,而交换的次数则跟初始数组的顺序有关，如果初始数组顺序为随机，则在最坏情况下数组元素将会交换N次，最好的情况是0次。选择排序的时间复杂度和空间复杂度分别为O(n2 ) 和 O(1)。

package algorithms;

publicabstractclass Sorter<E extends Comparable<E>> {

publicabstractvoid sort(E[] array,int from ,int len);

publicfinalvoid sort(E[] array)

{

sort(array,0,array.length);

}

protectedfinalvoid swap(E[] array,int from ,int to)

{

E tmp=array[from];

array[from]=array[to];

array[to]=tmp;

}

publicvoid sort(String helloString, int from, int len) {

// TODO Auto-generated method stub

}

}

package algorithms;

/**

* @author yovn

*

*/

publicclass SelectSorter<E extends Comparable<E>> extends Sorter<E> {

/* (non-Javadoc)

* @see algorithms.Sorter#sort(E[], int, int)

*/

@Override

publicvoid sort(E[] array, int from, int len) {

for(int i=0;i<len;i++)

{

int smallest=i;

int j=i+from;

for(;j<from+len;j++)

{

if(array[j].compareTo(array[smallest])<0)

{

smallest=j;

}

}

swap(array,i,smallest);

}

}

publicstaticvoid main(String[] args){

String[] myStringArray = new String[3];

String[] myStringArray1 = {"a","c","b"};

String[] myStringArray2 = new String[]{"a","b","c"};

SelectSorter<String> s1 = new SelectSorter<String>();

for(int i=0;i<3;i++){

System.out.println(myStringArray1[i]);

}

s1.sort(myStringArray1, 0, 3);

for(int i=0;i<3;i++){

System.out.println(myStringArray1[i]);

}

}

}

Output:

a

c

b

a

b

c