fix pattern matching exercise solution

[mustafapc]

my solution works on more complex Expressions and Operations than the old one

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[gribozavr]

Sorry, the change looks like a stylistic refactoring to me. What's the motivation?

[mustafapc]

@gribozavr the motivation is that the solution used on the tutorial only works on trees that's two or three operation deep and mine can work with complex trees, try a operation for example ((((4 + 7) * (8 - 2)) / ((9 + 14) * (16 - 4))) + 5) or something even deeper than this my solution would work and the one in tutorial won't