Find the first and last positions of elements in a sorted array

    public int[] searchRange(int[] nums, int target) {


        int [] num = new int[2];
        // When the array is empty 
        if (nums.length==0){
            num[0] = -1;
            num[1] = -1;
            return num;

        }
        // Binary search finds the corresponding element , If not, return -1
        int flag =  commonBinarySearch(nums,target);
        // Cannot find element in array 
        if (flag == -1) {
            num[0] = -1;
            num[1] = -1;
            return num;
        }
        // Set two pointers , Point to the beginning and end of the same element 
          int head = flag;
          int rear = flag;
          // When the border is reached, exit 
          while (head!=0){
//               If it is equal to the previous element , You also need to move head forward 
              if (nums[head-1]==nums[head]){
                  head --;
              }else {
//                   Not equal to the previous element , Find the leftmost boundary of the element 
                  break;
              }

          }
          
        // When the border is reached, exit 
        while (rear!=nums.length-1){
//               If it is equal to an element , You also need to move rear forward 
            if (nums[rear]==nums[rear+1]){
                rear ++;
            }else {
//                   Not equal to the previous element , Find the rightmost boundary of the element 
                break;
            }

        }
         num[0] = head;
         num[1] = rear;

        return num;
    }

    public static int commonBinarySearch(int[] arr,int key) {
        int low = 0;
        int high = arr.length - 1;
        int middle = 0;            // Definition middle

        if (key < arr[low] || key > arr[high] || low > high) {
            return -1;
        }

        while (low <= high) {
            middle = (low + high) / 2;
            if (arr[middle] > key) {
                // If it is larger than the keyword, the keyword is in the left area 
                high = middle - 1;
            } else if (arr[middle] < key) {
                // Smaller than the keyword, the keyword is in the right area 
                low = middle + 1;
            } else {
                return middle;
            }
        }

        return -1;        // Finally, I still haven't found , Then return to -1

        }