归并排序（升序）

1、算法思想

将若干个有序的序列进行两两合并，知道全部记录归并到一个序列。具体过程：将n个记录的待排序序列看成n个子序列，然后把他们两两归并，接着把长度为2的n/2个子序列再次归并，重复上述过程，直到有序为止。

2、算法实现

package test;

public class MergeSort {  

    public static void main(String[] args) {
        int[] n ={1,3,6,2,9,6,8,7};
        mergesort(n,0,n.length-1);
        for (int i : n) {
            System.out.print(i+"\t");
        }
    }  

    public static int[] mergesort(int[] nums, int low, int high) {
        int mid = (low + high) / 2;
        if (low < high) {
            mergesort(nums, low, mid);
            mergesort(nums, mid + 1, high);
            merge(nums, low, mid, high);
        }
        return nums;
    }  

    public static void merge(int[] nums, 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 (nums[i] < nums[j]) {
                temp[k++] = nums[i++];
            } else {
                temp[k++] = nums[j++];
            }
        }  

        while (i <= mid) {
            temp[k++] = nums[i++];
        }  

        while (j <= high) {
            temp[k++] = nums[j++];
        }  

        for (int k2 = 0; k2 < temp.length; k2++) {
            nums[k2 + low] = temp[k2];
        }
    }  

}