c# - the Time complexity of an algorithm -

i learning algorithms @ moment , wrote below code, finds if peak exists in 1 dimensional array. question how know time complexity of best case or average case?

the worst case time complexity (if element of array sorted) o(n), if array not sorted , has 10 elements code runs 5 loops result, half of array size, can tell me how write in algorithmic notation.

 int[] arr = { 1,5,2,7,8,9,4,6,7};  int peak;   int count = 0;  (int = 1; < arr.length - 1; i++)  {             peak = -1;             if (arr[i - 1] <= arr[i] && arr[i] >= arr[i + 1])             {                 peak = arr[i];                 i++;             }             else if (arr[i - 1] >= arr[i] && - 1 == 0)             {                 peak = arr[i - 1];             }             else if (arr[i + 1] >= arr[i] && + 1 == arr.length - 1)             {                 peak = arr[i + 1];             }              if (peak != -1)                 listbox1.items.add(peak.tostring());              count++; }  listbox1.items.add(count.tostring()); 

you still write o(n) because big-o worst case (which here linear time.) in case n of course array length. theoretically nothing inside loop exceeds o(1) (i'm going ignore list append , call constant.)

after noticing comments n/2 iterations. n/2 same saying (1/2)n or o((1/2)n) equivalent o(n) because ignore coefficients when doing bounding big-o.