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关于归并排序的原理这里不再赘述，直接附上代码

Java实现如下：

public class Solution6 {

    public static void  mergeSort(int[] arr){
        int[] tempArr = new int[arr.length];
        sort(arr,tempArr,0,arr.length-1);
    }

    private static void sort(int[] arr,int[] tempArr,int startIndex,int endIndex){
        if(endIndex <= startIndex) return;
        int middleIndex = startIndex + (endIndex - startIndex) / 2;

      	//分解
        sort(arr, tempArr, startIndex, middleIndex);
        sort(arr, tempArr, middleIndex + 1, endIndex);

      	//归并
        merge(arr, tempArr, startIndex, middleIndex, endIndex);
    }

    private static void merge(int[] arr, int[] temArr, int staIndex, int midIndex, int endIndex){
        for(int i = staIndex; i <= endIndex; i++) temArr[i] = arr[i];
        int left = staIndex;
        int right = midIndex+1;
        for(int i=staIndex; i<=endIndex; i++){
            if(left > midIndex) arr[i] = temArr[right++];
            else if(right > endIndex) arr[i] = temArr[left++];
            else if(temArr[right] < temArr[left]) arr[i] = temArr[right++];
            else  arr[i] = temArr[left++];
        }
    }
}

时间复杂度： O(nlogn)
空间复杂度： O(1)

补充：
arr[i++]; 先arr[i]再i自增
arr[++i]; 先i自增再arr[i]