归并排序

归并排序的时间复杂度为O(nlogn)

它采用的是分治法的思想，先将序列拆分为1个个子序列，直到每个序列的长度为1，此时每个序列都是有序的；然后将两个有序的子序列合并成为一个更大的子序列，递归此过程，直到子序列的长度和原序列相同，此时排序完成，图示如下：

具体代码如下：

import java.util.Arrays;/** * Created by 亮大王 on 2017/8/31. */public class no2 {    public static void main(String[] args) {        int a[] = {49, 38, 65, 97, 76, 13, 27};        System.out.println(Arrays.toString(a));        no2 nn = new no2();        nn.mergeSort(a, 0, a.length - 1);        System.out.println(Arrays.toString(a));    }    /**     * 二路归并     */    public void mergeArray(int a[], int low, int mid, int high) {        int temp[] = new int[high - low + 1];        int i = low;        int j = mid + 1;        int k = 0;        while (i <= mid && j <= high) {            if (a[i] < a[j])                temp[k++] = a[i++];            else                temp[k++] = a[j++];        }        while (i <= mid) {            temp[k++] = a[i++];        }        while (j <= high) {            temp[k++] = a[j++];        }        System.arraycopy(temp, 0, a, low, temp.length);    }    public void mergeSort(int a[], int low, int high) {        int mid = (high + low) / 2;        if (low < high) {            mergeSort(a, low, mid);            mergeSort(a, mid + 1, high);            mergeArray(a, low, mid, high);        }    }}