Merge pull request #20 from alberthu16/day26

Day 26: endian wars

Author: Albert Hu

day26/README.md

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+question of the day: https://codefights.com/challenge/ChcFLSa3rfJsKNgkC
+
+The fight on whether to store numbers starting with their most 
+significant digit or their least significant digit has been going 
+on for years. It even got a name and is called the Endian War by 
+some specialists.
+
+Joe Stoy in his (excellent, by the way) book "Denotational Semantics",
+tells the following story about Alan Turing: "...One early British
+computer had numbers running from right to left (because the spot
+on an oscilloscope tube runs from left to right, but in serial
+logic the least significant digits are dealt with first). Turing
+used to mystify audiences at public lectures when, quite by 
+accident, he would slip into this mode even for decimal arithmetic,
+and write things like 73+42=16...".
+
+You are given an expression that was presumably written by Alan Turing.
+Return true if it is a correct expression written in the little-endian
+decimal format, or return false otherwise.
+
+## Example
+
+For expression = 73+42=16, the output should be
+endianWar(expression) = true.
+
+In the little-endian decimal format, the expression becomes
+37 + 24 = 61, which is correct.
+
+For expression = "5+8=13", the output should be
+endianWar(expression) = false.
+
+In the little-endian decimal format, the result of the expression should
+be 31.
+
+## Ideas
+
+Let's break the expression string up and check the math. Reverse the
+two addends and sum them, then check against the reversed little-endian
+sum.
+
+`O(n)` to do the splits on the expression, `O(n)` to reverse the numbers
+and convert into ints. `O(1)` to check the math of the expression.
+
+## Code
+
+[Python](./endianWars.py)
+
+## Follow up

day26/endianWars.py

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+def endianWar(expression):
+    a, _ = expression.split("+")
+    b, c = _.split("=")
+    return int(a[::-1]) + int(b[::-1]) == int(c[::-1])
+
+def testEndianWar():
+    # little endian
+    assert endianWar("73+42=16")
+    assert endianWar("10+20=30")
+    assert endianWar("11+44=55")
+    assert endianWar("2466+7=9466")
+    assert endianWar("2+19=39")
+    assert endianWar("4+3=7")
+    assert endianWar("714+8=524")
+    assert endianWar("47+17=541")
+    assert endianWar("99+54962=44072")
+
+    # not little endian
+    assert not endianWar("8861+81209=90070")
+    assert not endianWar("5+8=13")
+    assert not endianWar("649+68072=68721")
+    assert not endianWar("65+72825=72890")
+    assert not endianWar("930+70374=71304")
+
+def main():
+    testEndianWar()
+
+if __name__ == "__main__":
+    return main()
+