Problem statement made by the author:
Given a list of N cities and the distances between M pairs of cities, what is the shortest possible route that visits each city exactly once and returns to the origin city? (N, M <= 2*10^5)
Intended solution by problem author: write a greedy or DP in $$$O(n^2)$$$
What actually happens during the contest: author discovers that this is actually an NP problem and cannot be solved for big N, M. Contest becomes unranted because of bad problem.
Conclusion: always check for impossible problems before the contest.