归并排序

归并排序的时间复杂度为
    T(n)=T(n/2)+T(n/2)+2n-1 = O(nlogn),
该算法优于选择排序、插入排序和冒泡排序。
import java.util.Arrays;
public class MergeSort {
/**@author liuwei
* @param args
*/
    public static void mergeSort(int[] data){
        System.out.println("开始排序：");
        sort(data,0,data.length-1);
    }
    /*将索引从left到right范围的数组元素进行归并排序
     * data 待排序数组
     * left 待排序数组的第一个元素索引
     * right 待排序数组的最后一个元素索引
     */
    private static void sort(int[] data, int left, int right) {
        // TODO Auto-generated method stub
        if(left<right){
            //找出中间索引
            int center=(left+right)/2;
            //对左边数组进行递归
            sort(data,left,center);
            //对右边数组进行递归
            sort(data,center+1,right);
            //合并
            merge(data,left,center,right);
           
        }
    }
    /*将两个数组进行归并，归并前两个数组已经有序，归并后依然有序
     * data 数组对象
     * left 左数组的第一个元素的索引
     * center 左数组的最后一个元素的索引，center+1是右数组第一个元素的索引
     * right 右数组的最后一个元素的索引
     */
    private static void merge(int[] data, int left, int center, int right) {
        // TODO Auto-generated method stub
        int [] tmpArr=new int[data.length];
        int mid=center+1;
        //third记录中间数组的索引
        int third=left;
        int tmp=left;
        while(left<=center&&mid<=right){
            //从两个数组中取出最小的放入中间数组
            if(data[left] < (data[mid])){
                tmpArr[third++]=data[left++];
            }else{
                tmpArr[third++]=data[mid++];
            }
        }
        //剩余部分依次放入中间数组
        while(mid<=right){
            tmpArr[third++]=data[mid++];
        }
        while(left<=center){
            tmpArr[third++]=data[left++];
        }
        //将中间数组中的内容复制回原数组
        while(tmp<=right){
            data[tmp]=tmpArr[tmp++];
        }
        System.out.println(Arrays.toString(data));
    }
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        int [] data={21,30,49,30,16,9,-16,10,25,18};
        System.out.println("排序之前：\n"+Arrays.toString(data));
        mergeSort(data);
        System.out.println("排序之后：\n"+Arrays.toString(data));
    }
}
结果输出：
排序之前：
[21, 30, 49, 30, 16, 9, -16, 10, 25, 18]
开始排序：
[21, 30, 49, 30, 16, 9, -16, 10, 25, 18]
[21, 30, 49, 30, 16, 9, -16, 10, 25, 18]
[21, 30, 49, 16, 30, 9, -16, 10, 25, 18]
[16, 21, 30, 30, 49, 9, -16, 10, 25, 18]
[16, 21, 30, 30, 49, -16, 9, 10, 25, 18]
[16, 21, 30, 30, 49, -16, 9, 10, 25, 18]
[16, 21, 30, 30, 49, -16, 9, 10, 18, 25]
[16, 21, 30, 30, 49, -16, 9, 10, 18, 25]
[-16, 9, 10, 16, 18, 21, 25, 30, 30, 49]
排序之后：
[-16, 9, 10, 16, 18, 21, 25, 30, 30, 49]