Auto comment: topic has been translated by timur_jalgasbaev (original revision, translated revision, compare)

unordered_map is basically O(1) for "random" inputs, but since the problemsetters/(people who make hacks) know that people will try using it, they can basically make an input that will "break" the unordered map and make it O(n) per access. Map on the other hand is always O(logn), and so its alot more safe to use.

I did the exact same thing. This article explains very well why the unordered_map fails and also discuss the solution to avoid such hacks in the future.

Auto comment: topic has been updated by timur_jalgasbaev (previous revision, new revision, compare).

Using gp_hash_table<int,int>mp works better than unordered_map because in the worst case, gp_hash_table operates in O(1), while unordered_map can degrade to O(n).Therefore, gp_hash_table is a better choise than unordered_map. If you use gp_hash_table, you need to include the following headers:

#include<ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;