归并排序

package Sort;

import java.util.Random;

public class Merge_Sort {
    final static int inf = Integer.MAX_VALUE;
    public static void main(String[] args) {
        //初始化数组
        int [] a = new int [1000];
        for(int i =0;i<a.length;i++){
            a[i] = new Random().nextInt(1000);
        }
        //排序前
        for(int i = 0;i< a.length;i++){
            if(i%20==0){
                System.out.println();
            }
            System.out.print(a[i]+" ");
        }
        System.out.println("\n");
        //调用归并排序算法
        Merge_sort(a,0,a.length-1);
        //排序后
        for(int i = 0;i<a.length;i++){
            if(i%20==0){
                System.out.println();
            }
            System.out.print(a[i]+" ");
        }
    }
    //归并排序
    public static void Merge_sort(int []A, int p, int r){
        if(p < r){
            int q = (p+r)/2;
            Merge_sort(A, p, q);//排序[p,q]
            Merge_sort(A, q+1, r);//排序[q+1,r]
            MERGE(A, p, q, r);//合并[p,q][q+1,r]数组
        }
    }
    //数组合并操作
    public static void MERGE(int []A, int p, int q, int r){
        int n1 = q-p+1;
        int n2 = r-q;
        int[]L = new int[n1+1];
        int[]R = new int[n2+1];
        //L数组 R数组赋值
        for(int i=0;i< n1;i++){
            L[i] = A[p+i];
        }
        for(int j = 0;j< n2;j++){
            R[j] = A[q+j+1];
        }
        //哨兵
        L[n1] = inf;
        R[n2] = inf;
        int i = 0;
        int j = 0;
        //合并数组并排序
        for(int k = p; k <= r; k++){
            if(L[i]<=R[j]){
                A[k] = L[i];
                i++;
            }else{
                A[k] = R[j];
                j++;
            }
        }
    }
}