PPi,PP comes before I and returns directly (item1).
iPP,PP comes after I and returns I directly.

PPi, if it is prepositioned, add 1 first, and then perform the subsequent operation, so add 1 here first, and then perform the return operation, so it will return 1
iPP. For post addition, the current operation will be performed first, and then the plus 1 operation will be performed, so here return first, then + 1, but return in a function will not be performed later, so it is directly returned to I