check-if-an-array-can-be-divided-into-pairs-whose-sum-is-divisible-by-k.cpp

//
#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ft first
#define sc second
#define pb push_back
#define ins insert
#define all(v) v.begin(), v.end()
#define Sort(v) sort(all(v))
#define s(a, n) sort(a, a + n)
#define f1(i,a,b) for(ll i=a;i<b;i++)
#define f2(i,a,b,c) for(ll i=a;i<b;i+=c)
#define f3(i,a,b) for(ll i=a;i>=b;i--)
#define f4(i,a,b,c) for(ll i=a;i>=b;i-=c)
#define jatin(t) int t; cin >> t; while(t--)
const int N = 1e5+2, MOD = 1e9+7;
//int array[N];
//cout << (condition ? "YES\n" : "NO\n");
// { Driver Code Starts
#include <bits/stdc++.h>
using namespace std;

 // } Driver Code Ends
class Solution {
  public:
    bool canPair(vector<int> nums, int k) {
        // Code here.
        unordered_map<int,int> mp;
        if(nums.size()%2!=0){
            return false;
        }
        for(int i=0;i<nums.size();i++){
            mp[nums[i]%k]++;
        }
        if(mp[0]%2==1){
            return false;
        }
       unordered_map<int,int> :: iterator it;
        for(it=mp.begin();it!=mp.end();it++){
            if(it->second!=mp[(k-it->first)%k]){
                    return false;
                }
        }
        return true;
    }
};

// { Driver Code Starts.
int main() {
    int tc;
    cin >> tc;
    while (tc--) {
        int n, k;
        cin >> n >> k;
        vector<int> nums(n);
        for (int i = 0; i < nums.size(); i++) cin >> nums[i];
        Solution ob;
        bool ans = ob.canPair(nums, k);
        if (ans)
            cout << "True\n";
        else
            cout << "False\n";
    }
    return 0;
}  // } Driver Code Ends