图解Java经典算法归并排序的原理与实现

public class MergeSort {
    //归并所需的辅助数组
    private static Comparable[] assist;
    //比较 v 是否小于 w
    public static boolean less(Comparable v,Comparable w){
        return v.compareTo(w) < 0;
    }
    //数组元素交换位置
    private static void swap(Comparable[] a,int i,int j){
        Comparable temp;
        temp = a[i];
        a[i] = a[j];
        a[j] = temp;
    }
    //排序
    public static void sort(Comparable[] a){
        //初始化辅助数组
        assist = new Comparable[a.length];
        int l = 0;
        int h = a.length - 1;
        sort(a,l,h);
    }
    private static void sort(Comparable[] a,int l,int h){
        if (h <= l){
            return;
        }
        //分组
        int mid = l +(h - l) / 2;
        //分别对每组数据排序
        sort(a,l,mid);
        sort(a,mid + 1,h);
        //对数组进行归并
        merge(a,l,mid,h);
    }
    //对数组进行归并
    private static void merge(Comparable[] a,int l,int mid,int h){
        //定义三个指针
        int i = l;
        int p1 = l;
        int p2 = mid + 1;
        //遍历，移动p1,p2指针，比较两处索引的值,小的放到辅助数组的对应索引处
        while (p1 <= mid && p2 <=h){
            if (less(a[p1],a[p2])){
                assist[i++] = a[p1++];
            }else {
                assist[i++] = a[p2++];
            }
        }
        //遍历数组，如果p1的指针没有走完，则顺序移动p1指针，把对应的元素放到辅助数组的对应索引处
        while (p1 <= mid){
            assist[i++] = a[p1++];
        }
        //遍历数组，如果p2的指针没有走完，则顺序移动p2指针，把对应的元素放到辅助数组的对应索引处
        while (p2 <= h){
            assist[i++] = a[p2++];
        }
        //把辅助数组中的元素拷贝到原数组中
        for (int j = l; j <= h; j++) {
            a[j] = assist[j];
        }
    }
}

public class MergeSortTest {
    public static void main(String[] args) {
        Integer[] arr = {5,6,3,1,8,7,2,4};
        MergeSort.sort(arr);
        System.out.println(Arrays.toString(arr));
    }
}
//排序前：{5,6,3,1,8,7,2,4}
//排序后：{1,2,3,4,5,6,7,8}