Useful tricks to remember while solving problems:

• In $\text{avg} = x$ problems on array $a$, you can do $a[i] = a[i] - x$ for all $i$, so that size of subarr/subseq becomes irrelevant. [https://codeforces.com/gym/104471/problem/C]
• For median problems, you can use "replace by $-1,$ $+1$" trick to estimate the median. [https://codeforces.com/problemset/problem/1486/D]
• To factorize large numbers upto $10^{18}$, we must use Pollard Rho's algorithm, which works in $\mathcal{O}(n^{1/4})$.
• Never compare floats/doubles with 128-bit integers in C++ (__int128). [https://codeforces.com/gym/101485/problem/K]
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• Stars and Bars problem, but with (common) upper bound: https://math.stackexchange.com/questions/553960/ [https://codeforces.com/gym/510395/problem/H]
• More tricks will be added here..