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Nice solution. How does one deduce a solution like this ?
Before peeking into the discussion forum, I was thinking that if l is 0 then the answer would be r ^ 0 = r. Hence, when l is not 0 the answer should have something to do with r ^ l. But I could not take my thought process any further.
For me what works is taking a few examples and trying to find a pattern. Once I find a pattern, I convince myself that it will work for all cases or try to find a test case that will fail. Once I find a test case that fails I'll try to modify the pattern or come up with a new pattern. Like in this case the pattern was the MSB that differs between the two numbers. Hope this helps.