Since you need to use ++n to use the increasing value in this expression.
See that in the expression tree it does not increment n , and then uses this value in the appendix because n++ returns n , but increments it for the next expression used.
However, ++n will actually return the added value of n for this expression.
Therefore, n++ + 5 gives 25 , while ++n + 5 gives 26 .
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