Java算法之排序

Java之冒泡排序

    public static void bubbleSort(int[] array){
        //3 1 5 8 2 9 4 6 7    n*(n-1)/2    n
        for(int i=array.length-1;i>0;i--) {
            boolean flag=true;
            for (int j = 0; j < i; j++) {
                if (array[j] > array[j + 1]) {
                    int temp = array[j];
                    array[j] = array[j + 1];
                    array[j + 1] = temp;
                    flag=false;
                }
            }
            if(flag){
                break;
            }
        }
    }  

Java之选择排序

  public static void selectSort(int[] array){
        for(int i=0;i<array.length-1;i++) {
            int index = i;
            for (int j = i+1; j < array.length; j++) {
                if (array[j] < array[index]) {
                    index = j;
                }
            }
            //{1,2,5,8,3,9,4,6,7};
            if(index!=i) {//如果已经是最小的，就不需要交换
                int temp = array[index];
                array[index] = array[i];
                array[i] = temp;
            }
        }
    }

Java之其他排序

public class ExampleUnitTest {
    @Test
    public void addition_isCorrect() throws Exception {
        int[] array=new int[]{1,7,4,9,3,2,6,5,8};
        int key=35;
        quickSort(array,0,array.length-1);

//        System.out.println(binarySearch(array,0,array.length,key));
        for (int i : array) {
            System.out.print(i+" ");
        }
    }
    /**
     * 二分查找
     */
    public static int binarySearch(int[] array,int fromIndex,int toIndex,int key){
        int low=fromIndex;
        int high=toIndex-1;
        while(low<=high){
            int mid=(low+high)/2;//取中间
            int midVal=array[mid];
            if(key>midVal){//去右边找
                low=mid+1;
            }else if(key<midVal){//去左边找
                high=mid-1;
            }else{
                return mid;
            }
        }
        return -(low+1);//low+1表示找不到时停在了第low+1个元素的位置
    }

    //快速排序     31  21  59  68  12  40
    //    x=31
    public static void quickSort(int[] array,int begin,int end){
        if(end-begin<=0) return;
        int x=array[begin];
        int low=begin;//0
        int high=end;//5
        //由于会从两头取数据，需要一个方向
        boolean direction=true;
        L1:
        while(low<high){
            if(direction){//从右往左找
                for(int i=high;i>low;i--){
                    if(array[i]<=x){
                        array[low++]=array[i];
                        high=i;
                        direction=!direction;
                        continue L1;
                    }
                }
                high=low;//如果上面的if从未进入，让两个指针重合
            }else{
                for(int i=low;i<high;i++){
                    if(array[i]>=x){
                        array[high--]=array[i];
                        low=i;
                        direction=!direction;
                        continue L1;
                    }
                }
                low=high;
            }
        }
        //把最后找到的值 放入中间位置
        array[low]=x;
        //开始完成左右两边的操作
        quickSort(array,begin,low-1);
        quickSort(array,low+1,end);
    }
    @Test
    public void test(){
        int[] array=new int[]{2,1,6,4,3,9,8,10,7,5};
//        merge(array,0,4,7);
        mergeSort(array,0,array.length-1);
        for (int i : array) {
            System.out.print(i+" ");
        }
    }
    public static void mergeSort(int array[],int left,int right){
        if(left==right){
            return;
        }else{
            int mid=(left+right)/2;
            mergeSort(array,left,mid);
            mergeSort(array,mid+1,right);
            merge(array,left,mid+1,right);
        }
    }

    //    0    4   7
    //    1  2  5  9 === 3  4  10  11
    public static void merge(int[] array,int left,int mid,int right){
        int leftSize=mid-left;
        int rightSize=right-mid+1;
        //生成数组
        int[] leftArray=new int[leftSize];
        int[] rightArray=new int[rightSize];
        //填充数据
        for(int i=left;i<mid;i++){
            leftArray[i-left]=array[i];
        }
        for(int i=mid;i<=right;i++){
            rightArray[i-mid]=array[i];
        }
        //合并
        int i=0;
        int j=0;
        int k=left;
        while(i<leftSize && j<rightSize){
            if(leftArray[i]<rightArray[j]){
                array[k]=leftArray[i];
                k++;i++;
            }else{
                array[k]=rightArray[j];
                k++;j++;
            }
        }
        while(i<leftSize){
            array[k]=leftArray[i];
            k++;i++;
        }
        while(j<rightSize){
            array[k]=rightArray[j];
            k++;j++;
        }
    }
}