归并排序及分析

归并排序的思想是将数组分成两部分，分别进行排序，然后归并起来。

public static void mergeSort(int[] arr){
        if(arr.length>1){
            int[] firstHalf = new int[arr.length/2];
            System.arraycopy(arr,0,firstHalf,0,arr.length/2);
            mergeSort(firstHalf);

            int secondLen = arr.length-arr.length/2;
            int[] secondHalf = new int[secondLen];
            System.arraycopy(arr,arr.length/2,secondHalf,0,secondLen);
            mergeSort(secondHalf);

            int[] temp = merge(firstHalf,secondHalf);
            System.arraycopy(temp,0,arr,0,temp.length);
        }
    }

    public static int[] merge(int[] list1,int[] list2){
        int[] list=new int[list1.length+list2.length];
        int current1=0;
        int current2=0;
        int current3=0;

        while (current1<list1.length&&current2<list2.length){
            if (list1[current1]<=list2[current2]){
                list[current3++]=list1[current1++];
            }
            else{
                list[current3++]=list2[current2++];
            }
        }
        while (current1<list1.length){
            list[current3++]=list1[current1++];
        }
        while (current2<list2.length){
            list[current3++]=list2[current2++];
        }
        return list;
    }