几种排序之归并排序

还是这个老哥写的好：https://blog.51cto.com/13733462/2115396

二路合并排序的基本思想是：对于两个有序表合并，初始时， 把含有n个结点的待排序序列看作有n个长度为1的有序子表所组成，将它们依次两两合并，得到长度为2的若干有序子表，再对这些子表进行两两合并，一直重复到长度为n，排序完成。

package Sort;
import java.util.Arrays;

public class MergeSort {
    
    public static void mergeSort(int [] arr){
        
        if(arr ==null||arr.length<2){
            return ;            
        }
        sortProcess(arr, 0, arr.length-1);
    }
    
    public static void sortProcess(int []arr,int L,int R){
        if(L==R){
            return;
        }
        int mid = L+((R-L)>>1);//等同于(L+R)/2
        sortProcess(arr, L, mid);
        sortProcess(arr, mid+1, R);
        merge(arr,L,mid,R);
    }
    
    public static void merge(int []arr,int L,int mid,int R){
        int[] help= new int[R-L+1];
        int i =0;
        int p1 = L;
        int p2 = mid+1;
        while(p1<=mid && p2<=R){
            if(arr[p1]<arr[p2]){
                help[i] = arr[p1];
                p1++;
            }
            else{
                help[i]=arr[p2];
                p2++;
            }
            i++;
        }
        while(p1<=mid){
            help[i++] = arr[p1++];
        }
        while(p2<=R){
            help[i++]=arr[p2++];
        }
        
        for(i=0;i<help.length;i++){//把排序好的help拷贝回原数组
            arr[L] = help[i];
            L++;
        }
        
    }
    public static void comparator(int[] arr) {
        Arrays.sort(arr);//系统排序绝对正确的方法
    }
    
    public static int[] generateRandomArray(int maxSize, int maxValue) {//随机数发生器
        //Math.random 产生一个double [0,1)
        
        int[] arr = new int[(int) ((maxSize + 1) * Math.random())];//产生一个随机长度的数组
        for (int i = 0; i < arr.length; i++) {
            arr[i] = (int) ((maxValue + 1) * Math.random()) - (int) (maxValue * Math.random());//产生一个随机数，两个随机数减一下
        }
        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 = 500000;
        int maxSize = 10;
        int maxValue = 100;
        boolean succeed = true;
        for (int i = 0; i < testTime; i++) {
            int[] arr1 = generateRandomArray(maxSize, maxValue);
            int[] arr2 = copyArray(arr1);
            mergeSort(arr1);
            comparator(arr2);
            if (!isEqual(arr1, arr2)) {
                succeed = false;
                break;
            }
        }
        System.out.println(succeed ? "Nice!" : "Fucking fucked!");

        int[] arr = generateRandomArray(maxSize, maxValue);
        printArray(arr);
        mergeSort(arr);
        printArray(arr);
        
    }

}