归并排序（Java）

归子莫

发布于：2020年6月27日

归并排序（Java）

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归并排序介绍

归并排序（MERGE-SORT）是利用归并的思想实现的排序方法，该算法采用经典的分治（divide-and-conquer）策略（分治法将问题分(divide)成一些小的问题然后递归求解，而治(conquer)的阶段则将分的阶段得到的各答案”修补”在一起，即分而治之)。

代码

package cn.guizimo.sort;


import java.util.Arrays;

public class MergetSort {
    public static void main(String[] args) {
        int arr[] = {8, 4, 5, 7, 1, 3, 6, 2};
        int temp[] = new int[arr.length];
        System.out.println("排序前");
        System.out.println(Arrays.toString(arr));
        mergeSort(arr, 0, arr.length - 1, temp);
        System.out.println("排序后");
        System.out.println(Arrays.toString(arr));

    }

    public static void mergeSort(int[] arr, int left, int right, int[] temp) {
        if (left < right) {
            int mid = (left + right) / 2;
            mergeSort(arr, left, mid, temp);
            mergeSort(arr, mid + 1, right, temp);
            merge(arr, left, mid, right, temp);
        }
    }

    public static void merge(int[] arr, int left, int mid, int right, int[] temp) {
        int i = left;
        int j = mid + 1;
        int t = 0;

        while (i <= mid && j <= right) {
            if (arr[i] <= arr[j]) {
                temp[t] = arr[i];
                t += 1;
                i += 1;
            } else {
                temp[t] = arr[j];
                t += 1;
                j += 1;
            }
        }

        while (i <= mid) {
            temp[t] = arr[i];
            t += 1;
            i += 1;
        }

        while (j <= right) {
            temp[t] = arr[j];
            t += 1;
            j += 1;
        }

        t = 0;
        int tempIndex = left;
        while (tempIndex <= right) {
            arr[tempIndex] = temp[t];
            t += 1;
            tempIndex += 1;
        }
    }
}

测试

速度测试

package cn.guizimo.sort;


import java.util.Arrays;

public class MergetSort {
    public static void main(String[] args) {
        int max = 80000;
        int[] arr = new int[max];
        for (int i = 0; i < max; i++) {
            arr[i] = (int)(Math.random() * 8000000);
        }
        int temp[] = new int[arr.length];
        long date1 = System.currentTimeMillis();
        mergeSort(arr, 0, arr.length - 1, temp);
        long date2 = System.currentTimeMillis();
        System.out.println("归并排序"+max+"数组的时间为："+(date2-date1));
    }

    public static void mergeSort(int[] arr, int left, int right, int[] temp) {
        if (left < right) {
            int mid = (left + right) / 2;
            mergeSort(arr, left, mid, temp);
            mergeSort(arr, mid + 1, right, temp);
            merge(arr, left, mid, right, temp);
        }
    }

    public static void merge(int[] arr, int left, int mid, int right, int[] temp) {
        int i = left;
        int j = mid + 1;
        int t = 0;

        while (i <= mid && j <= right) {
            if (arr[i] <= arr[j]) {
                temp[t] = arr[i];
                t += 1;
                i += 1;
            } else {
                temp[t] = arr[j];
                t += 1;
                j += 1;
            }
        }

        while (i <= mid) {
            temp[t] = arr[i];
            t += 1;
            i += 1;
        }

        while (j <= right) {
            temp[t] = arr[j];
            t += 1;
            j += 1;
        }

        t = 0;
        int tempIndex = left;
        while (tempIndex <= right) {
            arr[tempIndex] = temp[t];
            t += 1;
            tempIndex += 1;
        }
    }
}