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Project Euler is collection of puzzles based on math. The coding and designs skills are completely secondary to the math. The algorithms (except for computational number theory, computation geometry) are completely secondary. So it is knowledge of math that is barrier.

I keep learning math (and, sometimes, relatively new development in math) by solving problems. To start, I would recommend reading G.H. Hardy & E.M. Wright "An Introduction to the Theory of Numbers".

With LeetCode -- this is more traditional exercise in coding. I did not pay much attention to it. So, sorry, cannot provide any advice