归并排序

import java.lang.reflect.Array;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class Test {
	public static int[] sort(int[] nums, int low, int high) {
		//System.out.println(Arrays.toString(nums));
		
		int mid = (low + high) / 2;
		System.out.println(low + " " + mid + " " +  high);
		if (low < high) {
			// 左边
			sort(nums, low, mid);
			// 右边
			sort(nums, mid + 1, high);
			// 左右归并
			
			System.out.println("beg merge --- " + low + " " + mid + " " +  high);
			merge(nums, low, mid, high);
			System.out.println(Arrays.toString(nums));
		}
		 
		return nums;
	}

	public static void merge(int[] nums, int low, int mid, int high) {
		int[] temp = new int[high - low + 1];
		int i = low;     // 左指针
		int j = mid + 1; // 右指针
		int k = 0;       // 临时数组指针

		// 把较小的数先移到新数组中
		// mid作为分界点比较
		while (i <= mid && j <= high) {
			if (nums[i] < nums[j]) {
				temp[k++] = nums[i++];
			} else {
				temp[k++] = nums[j++];
			}
		}
		
		System.out.println("1" + Arrays.toString(temp));

		// 把左边剩余的数移入数组
		while (i <= mid) {
			temp[k++] = nums[i++];
		}
		
		System.out.println("2 " + Arrays.toString(temp));

		// 把右边边剩余的数移入数组
		while (j <= high) {
			temp[k++] = nums[j++];
		}
		System.out.println("3 " + Arrays.toString(temp));

		// 把新数组中的数覆盖nums数组
		for (int k2 = 0; k2 < temp.length; k2++) {
			nums[k2 + low] = temp[k2];
		}
	}

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		int[] nums = {5, 4, 3, 2, 1};// 6, 9, 0, 5, 4 };
		System.out.println(Arrays.toString(nums));

		sort(nums, 0, nums.length - 1);
		System.out.println(Arrays.toString(nums));

		Integer[] a = new Integer[] { 3, 2, 32, 3 };
		List<Integer> list = (List<Integer>) Arrays.asList(a);

		Collections.sort(list);

		for (int i = 0; i < list.size(); i++) {
			System.out.println(list.get(i));
		}

		System.out.println("test");
	}

}

Collections.sort(list); 里使用的归并排序
其实就是对数组递归拆分两个数组，或者更多，然后几个数组进行归并，然后依次添加到临时数组中， 临时数组最后也是有序的，然后再用临时数组来修改原数组.