归并排序Java代码实现

归并排序复习：

结论：归并排序时间复杂度为O(nlgn),额外空间复杂度为O(n)，实现可以做到稳定；

核心思想：典型的分冶策略思想：

第一步：拆分：递归对半拆分无序数组为无数的子数组；

第二步：排序：将子数组排好序；

第三步：合并：将子数组合并为和原先一样的长度的大数组；排序结束；

代码实现：

package com.cmbc.test1;

import java.util.Arrays;

/**
 * 归并排序
 * @author itqcy
 *
 */
public class MergeSortion {
    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-l)>>1);
        mergeSort(arr,l,mid);
        mergeSort(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 count = 0;
        int p1 = l;
        int p2 = mid+1;
        while(p1<=mid&&p2<=r){
            help[count++] = arr[p1]<arr[p2]?arr[p1++]:arr[p2++];
        }
        while(p1<=mid){
            help[count++] = arr[p1++];
        }
        while(p2<=r){
            help[count++] = arr[p2++];
        }
        for(int i = 0;i<help.length;i++){
            arr[l+i] = help[i];
        }
    }
    
    public static void printArray(int[] arr) {
        if (arr == null) {
            return;
        }
        for (int i = 0; i < arr.length; i++) {
            System.out.print(arr[i] + " ");
        }
        
    }
    
    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;
    }

    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 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;
    }
    
    public static void main(String[] args) {
        int testTime = 500000;
        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);
            mergeSort(arr1);
            Arrays.sort(arr2);
            if (!isEqual(arr1, arr2)) {
                succeed = false;
                printArray(arr1);
                printArray(arr2);
                break;
            }
        }
        System.out.println(succeed ? "归并排序结果正确!" : "错误!");

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