Question

what do you mean by the complexity of algorithm

Answer 1

Complexity of an algorithm is a measure of the amount of time and/or space required by an algorithm for an input of a given size (n).
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Answer 2

Time complexity of an algorithm signifies the total time required by the program to run till its completion.

The time complexity of algorithms is most commonly expressed using the big O notation. It's an asymptotic notation to represent the time complexity.