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package interview;

import java.util.Arrays;

/**

* Created by chengxiao on 2016/12/8.

*/

public class MergeSort {

public static void main(String []args){

int []arr = {9,8,7,6,5,4,3,2,1};

sort(arr);

System.out.println(Arrays.toString(arr));

}

public static void sort(int []arr){

int []temp = new int[arr.length];//在排序前，先建好一个长度等于原数组长度的临时数组，避免递归中频繁开辟空间

sort(arr,0,arr.length-1,temp);

}

private static void sort(int[] arr,int left,int right,int []temp){

if(left<right){

int mid = (left+right)/2;

sort(arr,left,mid,temp);//左边归并排序，使得左子序列有序

sort(arr,mid+1,right,temp);//右边归并排序，使得右子序列有序

merge(arr,left,mid,right,temp);//将两个有序子数组合并操作

}

}

private static void merge(int[] arr,int left,int mid,int right,int[] temp){

int i = left;//左序列指针

int j = mid+1;//右序列指针

int t = 0;//临时数组指针

while (i<=mid && j<=right){

if(arr[i]<=arr[j]){

temp[t++] = arr[i++];

}else {

temp[t++] = arr[j++];

}

}

while(i<=mid){//将左边剩余元素填充进temp中

temp[t++] = arr[i++];

}

while(j<=right){//将右序列剩余元素填充进temp中

temp[t++] = arr[j++];

}

t = 0;

//将temp中的元素全部拷贝到原数组中

while(left <= right){

arr[left++] = temp[t++];

}

}

}