Sometimes we see problems where a seemingly naive algorithm — for example simple brute force — is actually the correct solution. Mostly, I mean problems where, due to some clever observations, the complexity of the brute force algorithm is greatly reduced.
For example, in a recent contest we had 1168B - Good Triple. You can notice that any string of length at least 9 contains a "good triple", which means a brute force is sufficient here and runs in $$$O(n)$$$.
Or 1028F - Make Symmetrical where you can notice that on any given circle, there are not too many lattice points.
Randomized input is also a good source of these. In 896C - Willem, Chtholly and Seniorious you can observe that after a bit of time, most adjacent elements of the array are equal and write something seemingly naive based on that.
What are some other examples of problems where a stupid brute force is actually the correct solution?
problem Ehab and Expected XOR Problem
my code complexity was $$$O(2^{2n})$$$ which passed fast because it finds next number with low number of iterations so actual runtime of the code wasn't slow
Can you prove even that this solution produces maximal length, without using ideas from real solution?
1073D - Берляндская ярмарка brute force solution is correct because given cost reduces very fast on every iteration.
It often happens in problems where you need to print the answer with some precision. If something is unlikely to happen, don't compute it.
For example this one: https://codeforces.me/problemset/problem/559/D
https://www.codechef.com/GW19MOS/problems/MEANPROB
This is from a ICPC regional. The bruteforce algorithm works
Maybe just for me but I came up with the solution of 1361F in half a minute.
PS: my real color is orange
Because the problem setter is also stupid sometime.