There are many contests which require participants to solve a problem which is loosely based on a concept that is maybe similar to a famous problem.

For example, a problem is based on Longest Increasing Subsequence. Is it allowed for the participant to copy (fully copy a function with no changes done to variable names) from third-party sites and then proceed to solve the problem.

Lets say a problem involves finding GCD, so can I simply copy-paste a function which takes 2 inputs and returns the GCD? Will my submission be Accepted? Or will it be considered as Plagiarism ?