This week's book giveaway is in the Programmer Certification forum. We're giving away four copies of OCP Oracle Certified Professional Java SE 11 Programmer I Study Guide: Exam 1Z0-815 and have Jeanne Boyarsky & Scott Selikoff on-line! See this thread for details.
"Time complexity" is a rather vague term and can refer to different things. For instance, there is "worst case time complexity", "best case time complexity" and "average case time complexity". Big O can be used to express an upper bound on any of these. If you don't explicitly state what case Big O expresses an upper bound for, people will assume Big O means "upper bound to the worst case time complexity".
You can also use Big O to express an upper bound to space complexity for a specific case. You can use Big Omega to express a lower bound on either time or space complexity for a specific case.
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