归并排序

它的思想是将一个无序的数组先用递归进行二分，当分解为无限小时，即为单个整数时，再对其进行排列。得出有序的数组

代码如下：

package com.jll.sort;

public class MergeSort {

    public MergeSort() {

    }

    public void merge(int[] a, int start, int finish) {
        if (start == finish)
            return;
        else {
            int mid = (start + finish) / 2;

            merge(a, start, mid);

            merge(a, mid + 1, finish);

            sort(a, start, mid+1 , finish);
        }
    }

    public void sort(int[] a, int begin, int mid, int end) {
        int middle = mid - 1;
        int low = begin;
        int high = end;
        int b[] = new int[a.length];
        int count = 0;
        while (begin <= middle && mid <= end) {
            if (a[begin] <= a[mid]){
                b[count++] = a[begin++];
            } else {
                b[count++] = a[mid++];
            }
        }

        while (begin <= middle) {
            b[count++] = a[begin++];
        }
        while (mid <= end) {
            b[count++] = a[mid++];
        }

        for (int i = low; i <= high; i++) {
            a[i] = b[i-low];
        }
    }

    public static void main(String[] args) {
        MergeSort ms = new MergeSort();
        int a[] = new int[10];
        for (int i = 0; i < 10; i++) {
            a[i] = (int) (Math.random() * 100 + 1);
        }

        System.out.println("a:");
        for (int i = 0; i < a.length; i++) {
            System.out.print(a[i] + " ");
        }

        ms.merge(a, 0, 9);

        System.out.println();
        System.out.println("a:");
        for (int i = 0; i < a.length; i++) {
            System.out.print(a[i] + " ");
        }
    }
}

里面的代码需要仔细看才能明白其中的道理