归并排序

www.acwing.com/activity/co…

参考acwing的这一题

归并排序的核心概念；

1. 确定中点
2. 递归子问题
3. 归并

package org.acwing.basic;

import java.util.Arrays;

public class MergeSort {


    public static void main(String[] args) {


        int[] arr = {3, 1, 2, 4, 5};

        mergeSort(arr, 0, arr.length - 1);

        System.out.println(Arrays.toString(arr));

    }

    private static void mergeSort(int[] arr, int l, int r) {

        if (l >= r) {
            return;
        }

        int mid = l + r >> 1;
        mergeSort(arr, l, mid);
        mergeSort(arr, mid + 1, r);

        int[] tmp = new int[r - l + 1];
        int i = l, j = mid + 1;
        int k = 0;

        while (i <= mid && j <= r) {
            if (arr[i] <= arr[j]) {
                tmp[k++] = arr[i++];
            } else {
                tmp[k++] = arr[j++];
            }
        }

        while (i <= mid) {
            tmp[k++] = arr[i++];
        }

        while (j <= r) {
            tmp[k++] = arr[j++];
        }

        // 注意这里要归并回去！
        for (i = l, j = 0; i <= r; i++, j++) {
            arr[i] = tmp[j];
        }


    }
}