Create Boolean_expression.dart

[RimjhimGupta]

This program is for the evaluation of the Boolean expression where a string consists of only 0, 1, A, B, C where A = AND B = OR C = XOR The evaluation is done from left to right. The length of string will be odd. It will always be a valid string. This pull request is for Hacktober Fest 2020.

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• [ ] I have read CONTRIBUTING.md.
• [ ] This pull request is all my own work -- I have not plagiarized.
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[RimjhimGupta]

@Parowicz Sir, I request you to please help me in resolving the failed check.

[Parowicz]

Tests are failing because

1. evaluateBoolExpr("0A1") returns '1' ==> false and true != true
2. evaluateBoolExpr("1B0") returns '0' ==> true or false != false
3. evaluateBoolExpr("1C0") returns '0' ==> true xor false != false

[RimjhimGupta]

@Parowicz Sir, regarding the failing of tests, I had already specified in the description that evaluation is done from left to right only. But, you are trying to evaluate it from right to left. Please consider this Sir. Thank you.

[Parowicz]

Those operations are commutative https://en.wikipedia.org/wiki/Boolean_algebra#Laws In your code evaluateBoolExpr("1A0") is different from evaluateBoolExpr("0A1"). Evaluation strategy should not invalidate result.