归并排序

package com.cisco.www.test;

import java.util.Arrays;

/**
 * 归并排序
 * 算法思想：分治
 * 时间复杂度：O(nlogn)
 * 空间复杂度O(N)
 *
 */
public class MergeSort {
    public static void mergeSort(int[] arr){
        if(arr==null||arr.length<2){
            return;
        }
        mergeSort(arr,0,arr.length-1);
    }

    public static void mergeSort(int[] arr, int l, int r) {
        if(l==r){
            return;
        }
        int mid = (l+r)/2;
        mergeSort(arr,l,mid);
        mergeSort(arr,mid+1,r);
        merge(arr,l,mid,r);
    }

    private static void merge(int[] arr, int l, int m, int r) {
        int[] help = new int[r-l+1];
        int i = 0 ;
        int p1 = l;
        int p2 = m+1;
        while (p1<=m&&p2<=r){
            help[i++] = arr[p1]<arr[p2]?arr[p1++]:arr[p2++];
        }
        while (p1<=m){
            help[i++]=arr[p1++];
        }
        while (p2<=r){
            help[i++] = arr[p2++];
        }
        for(i = 0 ; i<help.length;i++){
            arr[l+i] = help[i];
        }
    }

    //test
    public static void main(String[] args){
        int testTime = 1000000;
        int size = 100;
        int value =100;
        boolean succeed = true;
        for(int i = 0 ; i <testTime ; i++){
            int[] arr1 = generateRandomArray(size,value);
            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(size,value);
        printArray(arr);
        mergeSort(arr);
        printArray(arr);
    }

    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;
    }

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

    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;
    }

    //test
    public static int[] generateRandomArray(int size, int value) {
        //生成长度为0-100之间的随机数组
        int[] arr = new int[(int)(Math.random()*(size+1))];
        for(int i =0  ; i<arr.length;i++){
            arr[i] = (int)(Math.random()*(value+1)) - (int)(Math.random()*value);
        }
        return arr;
    }
    //
    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();
    }

}