# 选择排序

``````时间复杂度为O(n^2)
``````
``````import java.util.Arrays;

public class Code_02_SelectionSort {
public static void selectionSort(int []arr){
if(arr==null||arr.length<2){
return;
}
for(int i=0;i<arr.length-1;i++){
int minIndex=i;
for(int j=i+1;j<arr.length;j++){
minIndex=arr[j]<arr[minIndex]?j : minIndex;
}
swap(arr,i,minIndex);
}
}

private static void swap(int[] arr, int i, int j) {

int temp=arr[i];
arr[i]=arr[j];
arr[j]=temp;
/*
java这样为什么会出错?
arr[i]+=arr[j];
arr[j]=a[i]-arr[j];
arr[j]=arr[i]-arr[j];
*/
}

//for test
public static void comparator(int []arr){
Arrays.sort(arr);
}

//for test
public static int []generateRandomArray(int maxSize,int maxValue){
int []arr=new int[(int)((maxSize+1)*Math.random())];
for (int i = 0; i < arr.length; i++) {
arr[i]=(int)((maxValue+1)*Math.random())-(int)((maxValue)*Math.random());

}
return arr;
}
//for test
public static int[]copyArray(int []arr){
if(arr==null){
return null;
}
int []res=new int[arr.length];
for (int i = 0; i < arr.length; i++) {
res[i]=arr[i];

}
return res;
}
//for test
public static boolean isEqual(int []arr1,int []arr2 ){
if((arr1==null&&arr2!=null)||(arr1!=null&&arr2==null)){
return false;
}
if (arr1==null&&arr2==null){
return true;
}
if(arr1.length!=arr2.length){
return false;
}
for (int i = 0; i < arr1.length; i++) {
if(arr1[i]!=arr2[i]){
return false;
}

}
return true;

}
//for test
public static void printArray(int []arr){
if (arr==null){
return;
}
for (int i = 0; i < arr.length; i++) {
System.out.print(arr[i]+" ");

}
System.out.println();
}

public static void main(String[] args) {
int testTime=5000000;
int maxSize=100;
int maxValue=100;
boolean succeed=true;
for (int i = 0; i < testTime; i++) {
int []arr1=generateRandomArray(maxSize,maxValue);
int []arr2=copyArray(arr1);
selectionSort(arr1);
comparator(arr2);
if(!isEqual(arr1,arr2)){
succeed=false;
printArray(arr1);
printArray(arr2);
break;
}

}
System.out.println(succeed?"Nice !":"Fucking fucked !");
int []arr=generateRandomArray(maxSize,maxValue);
printArray(arr);
selectionSort(arr);
printArray(arr);

}

}

``````
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