归并排序

package leetcode.sort;

import java.util.Arrays;
import java.util.Random;

/**
 * Created by blank on 2015-09-22 上午11:42.
 */
public class MergeSort {


    public static void main(String[] args) {
        int[] arr = new int[20];
        for (int i = 0; i < 20; i++) {
            arr[i] = new Random().nextInt(100);
        }
        mergeSort(arr, 0, 19);
        System.out.println(Arrays.toString(arr));
    }


    public static void mergeSort(int[] num, int left, int right) {
        if (num == null || left >= right) {
            return;
        }
        int mid = (left + right) >>> 1;
        mergeSort(num, left, mid);
        mergeSort(num, mid + 1, right);
        merge(num, left, mid, right);
    }

    public static void merge(int[] num, int left, int mid, int right) {
        if (num == null) {
            return;
        }
        int[] copy = num.clone();
        int i, j, k = left;
        for (i = left, j = mid + 1; i <= mid && j <= right; k++) {
            if (copy[i] < copy[j])
                num[k] = copy[i++];
            else
                num[k] = copy[j++];
        }
        while (i <= mid) {
            num[k++] = copy[i++];
        }
        while (j <= right) {
            num[k++] = copy[j++];
        }
    }

}