5 归并排序

package com.sort.s05mergesort;
import java.util.Arrays;
/**
 * @Author black_zhou
 */
public class MergeSort {
    public int[] mergeSort(int[] arrays){
        if(null == arrays || arrays.length < 2){
            return arrays;
        }
        int mid = arrays.length/2;
        int[] left = Arrays.copyOfRange(arrays,0,mid);
        int[] right = Arrays.copyOfRange(arrays,mid,arrays.length);
        return merge(left,right);
    }
    private int[] merge(int[] left,int[] right){
        // 如果拆分后的数组只有一个元素后，最后调用mergeSort()，则会将该单个元素的数组返回，这里就是递归的终点
        int[] leftsort = mergeSort(left);
        int[] rightsort = mergeSort(right);
        // 递归的终点结束后，然后依次又往上层返回结果
        // 这里开始就将排好序的两个子数组合并为一个大的数组
        int length = leftsort.length + rightsort.length;
        int[] sortarray = new int[length];
        int l=0,r=0,i=0;
        while(l<leftsort.length && r<rightsort.length){
            if(leftsort[l]<rightsort[r]){
                sortarray[i] = leftsort[l];
                l++;
            }else{
                sortarray[i] = rightsort[r];
                r++;
            }
            i++;
        }
        while(l<leftsort.length){
            sortarray[i] = leftsort[l];
            l++;
            i++;
        }
        while(r<rightsort.length){
            sortarray[i] = rightsort[r];
            r++;
            i++;
        }
        return sortarray;
    }
    public void println(int [] a) {
        for(int x:a) {
            System.out.print(" "+x);
        }
        System.out.println("");
    }
    public static void main(String[] args) {
        int[] nums = {5, 3, 4, 6, 7, 1, 2, 8, 4, 10, 100, 50, 30, 22, 34, 55, 43, 12, 31};
        MergeSort sort = new MergeSort();
        sort.println(sort.mergeSort(nums));
    }
}