0082 分治法实现归并排序

package sort;

public class MergeSorting {
    public static void mergeSort(int[] nums, int start, int end){
        if (start == end) //终止条件（细品）
            return;

        int mid = (start + end)/2; //数组二分
        mergeSort(nums, start, mid); //对数组左半边排序
        mergeSort(nums, mid + 1, end);//对数组右半边排序
        merge(nums, start, mid, end);//合并两个有序数组

    }
    //合并两个有序数组
    private static void merge(int[] nums,int start, int mid, int end){
        if (start > end)
            return;
        int i = start;
        int j = mid + 1;

        int[] res = new int[nums.length];
        int k = start;  //新数组的下标
        while (i <= mid && j <= end ){
            if ( nums[i] <= nums[j])//当左边数组小于右边数组就一直将左边复制到新数组
                res[k++] = nums[i++];  //细品，此处的处理尤为高效精简！
            else //当右边数组小于左边数组就一直将右边复制到新数组
                res[k++] = nums[j++];
        }
        //当i或j中一个到达右边界上述循环退出
        //此时将未复制的数组片段全部复制到新数组
        while (i <= mid)
            res[k++] = nums[i++];
        while (j <= end)
            res[k++] = nums[j++];

        //将有序数组复制到nums
        for (int h = start; h <= end; h++)
            nums[h] = res[h];

    }


    public static void main(String[] args){
        int[] nums = {2,0,1,9,1,0,2,8};
        mergeSort(nums, 0, nums.length - 1);
        for (int num: nums)
            System.out.print(num+" ");
    }
}