You need to compile with gnu(https://gcc.gnu.org/). If you need only for codeforces, you can simply add this snippet to your code when you need to use unordered_map. I think this is the simplest solution.

#ifdef ONLINE_JUDGE

#pragma GCC optimize ("O3")
#pragma GCC target ("sse4")

#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/rope>

using namespace __gnu_pbds;
using namespace __gnu_cxx;

template <class T> using Tree = tree<T, null_type, less<T>, rb_tree_tag,tree_order_statistics_node_update>;

namespace mapOp {
    const int tmp = chrono::high_resolution_clock::now().time_since_epoch().count();

    template<class T> struct hsh {
        size_t operator()(const T& x) const { 
            return hash<T>{}(x)^tmp; // avoid anti-hash tests?
        }
    };

    template<class a, class b> using um = gp_hash_table<a,b,hsh<a>>;

    template<class a, class b> b get(um<a,b>& u, a x) {
        if (u.find(x) == u.end()) return 0;
        return u[x];
    }
}
using namespace mapOp;

#else

#define um unordered_map
template<class a, class b> b get(unordered_map<a,b>& u, a x) {
    if (u.find(x) == u.end()) return 0;
    return u[x];
}
#endif

um<int,int> used;

So I finally managed to do this in a better way on Windows. I used Windows Subsystem for Linux (the Ubuntu version), and now I test locally with no hassles, plus I can use unix commands.

In short, what you do is: you go to Windows settings, activate Windows Subsystem for Linux, then download whatever flavor you want from the Windows Store. Then just run that application, wait for it to install, install gcc, and you're done. Just navigate to /mnt/c/ to actually access your files on Windows and compile them. Then you'll be able to use these libraries with 0 issues.

I have MinGW-G++ 6.3.0. There was the file named "hash_standard_resize_policy_imp.hpp0000644" in my case. I had success when renamed it to match the name of the missing one. Other way is to paste contents of "hash_policy.hpp" and replace the missing include with the contents of "hash_standard_resize_policy_imp.hpp0000644". In the end it looks like this (https://paste.ubuntu.com/p/zn8MFYPxtv/).

for MacOS: brew install gcc is good enough

Can anyone please suggest a solution to this for windows??

Has anyone used pbds in visual studio on windows ?if yes, how?

Maybe you should try a newer version of gcc?

it seems does not work with the latest version of VSCode? I can include bits/stdc++.h but it can not recognize pbds headers. Any hints?

(FOR WINDOW USERS) : Window users might face problem importing pbds library . To fix it follow the following steps:

Step 1: Go to directory  where MinGW is installed.

     Step 2: Navigate to \lib\gcc\mingw32\8.2.0\include\c++\ext\pb_ds\detail\resize_policy

     Step 3: Rename “hash_standard_resize_policy_imp.hpp0000644” to “hash_standard_resize_policy_imp.hpp”

I have MinGW-G++ 6.3.0. and I used these steps, it worked for me.