0082 分治法實現歸併排序

package sort;

public class MergeSorting {
    public static void mergeSort(int[] nums, int start, int end){
        if (start == end) //終止條件（細品）
            return;

        int mid = (start + end)/2; //數組二分
        mergeSort(nums, start, mid); //對數組左半邊排序
        mergeSort(nums, mid + 1, end);//對數組右半邊排序
        merge(nums, start, mid, end);//合併兩個有序數組

    }
    //合併兩個有序數組
    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;  //新數組的下標
        while (i <= mid && j <= end ){
            if ( nums[i] <= nums[j])//當左邊數組小於右邊數組就一直將左邊複製到新數組
                res[k++] = nums[i++];  //細品，此處的處理尤爲高效精簡！
            else //當右邊數組小於左邊數組就一直將右邊複製到新數組
                res[k++] = nums[j++];
        }
        //當i或j中一個到達右邊界上述循環退出
        //此時將未複製的數組片段全部複製到新數組
        while (i <= mid)
            res[k++] = nums[i++];
        while (j <= end)
            res[k++] = nums[j++];

        //將有序數組複製到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+" ");
    }
}