left + (right - left) / 2
is the same as
(right + left) / 2
They both take the average of right and left. The purpose of using the former one is to prevent integer overflows. If right + left is bigger than the variable can carry, even though this variable can carry their average, the result will be wrong, because right + left will cause overflow.
On the other hand, first calculating (right - left) / 2 and adding left prevents overflow.
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