https://www.bilibili.com/video/BV1Zt411o7Rn?p=25

參考：這個視訊借鑑的歸併寫法

主要邏輯在對排好後的兩個陣列進行歸併邏輯，就是有一個指向第一個陣列的指標i和指向第二個陣列的指標j

進行比較並將小的結果放進臨時陣列，直到一方遍歷完成為止，然後再把沒有遍歷完的陣列以此進入陣列即可

 private static void mergeSort(int[] arr, int start , int end){
        if(start >= end){
            return;
        }
        int mid = (end + start) / 2;
        mergeSort(arr,start,mid);
        mergeSort(arr,mid
 
+1,end);
        merge(arr,start,mid ,end);
    }

    private static void merge(int[] arr, int start, int mid, int end) {
        int[] temp = new int[end - start + 1];
        int leftIndex = start;
        int rightIndex = mid + 1;
        int index = 0;
        while (leftIndex <= mid && rightIndex <= end ){
            
 
if(arr[leftIndex] < arr[rightIndex]){
                temp[index] = arr[leftIndex];
                leftIndex++;
            }else {
                temp[index] = arr[rightIndex];
                rightIndex++;
            }
            index++;
        }
        while (leftIndex <= mid){
            temp[index] 
 
= arr[leftIndex];
            leftIndex++;
            index++;
        }
        while (rightIndex <= end){
            temp[index] = arr[rightIndex];
            rightIndex++;
            index++;
        }

        for (int i = 0; i < temp.length; i++) {
            arr[start + i] = temp[i];
        }
    }