准备面试手写了吗（排序算法，递归，设计模式）

准备面试手写了吗（排序算法，设计模式，递归）

//冒泡排序 
    public void maoPaoPX(){
          
   
    
        int[] arr = new int[]{
          
   12,4,54,57,87,3,41,1,3,4,1,3,4,31,2};
        
        for (int i = 0; i < arr.length-1; i++) {
          
   
            for (int j = 0; j < arr.length-1-i; j++) {
          
   
                if (arr[j]<arr[j+1]){
          
   
                    int temp = arr[j];
                    arr[j] = arr[j+1];
                    arr[j+1] = temp;
                }
            }
        }
        //冒泡大到小
        System.out.println(Arrays.toString(arr));
        
        //工具排序小到大
        Arrays.sort(arr);
        System.out.println(Arrays.toString(arr));
    }

//选择排序  
    public void xuanZePX(){
          
   
    
        int[] arr={
          
   1,3,2,45,65,33,12};
        
        for (int i = 0; i < arr.length - 1; i++) {
          
   
            int index = i;
            int j;
            // 找出最小值得元素下标
            for (j = i + 1; j < arr.length; j++) {
          
   
                if (arr[j] < arr[index]) {
          
   
                    index = j;
                }
            }
            int tmp = arr[index];
            arr[index] = arr[i];
            arr[i] = tmp;
            System.out.println(Arrays.toString(arr));
        }
    }

//递归求和  
public class DiguiTest {
          
   

    public static void main(String[] args) {
          
   
        System.out.println(sumDg(100));
    }
    
    private static int sumDg(int i) {
          
   
        if (i == 0) {
          
   
            return 0;
        }
        return i+sumDg(i-1);
    }
}

//单例设计模式
public class Singleton {
          
   

    private volatile static Singleton instance = null;
    
    private Singleton(){
          
   }
    
    public static Singleton getInstance(){
          
   
        if(instance == null){
          
   
            synchronized(Singleton.class){
          
   
                if(instance == null){
          
   
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}