Money Matters uVA 11690 gives "Wrong Answer" and "Time Limit Exceeded" after multiple tries

Question

Here is the link to the problem: prob https://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&page=show_problem&problem=2737

Following is my implementation. I tried their sample test case and multiple others and they all give the correct answers. But when I submit I keep getting Wrong Answer. Before this I implemented this with a vector of vectors and I got "Time Limit Exceeded". I am not sure what's wrong with code, I would appreciate it if someone can help be identify the test case my solution is failing.

#include <iostream>
#include <vector>
#include <string>
#include <utility>
#include <algorithm>
#include <unordered_set>
#include <unordered_map>

using namespace std;

struct Node{
    Node* parent; 
    int rank;
    int data;
};


class DisjointSet {
    public:

    unordered_map<int, Node*> elms;

    void makeSet(int data){
        Node* temp  = new Node();
        temp->data = data; 
        temp->rank = 0; 
        temp->parent = temp;
        pair<int, Node*> p(data, temp);
        elms.insert(p);
    }

    int findSet(int a){
        return findSet(elms[a])->data;
    }

    Node* findSet(Node* a){
        if(a == nullptr) return a;
        if( a == a->parent ) return a->parent;
        a->parent  = findSet(a->parent);
        return a->parent;
    }

    void unionS(int a, int b){
        Node* n1 = elms[a];
        Node* n2 = elms[b];
        Node* p1 = findSet(n1);
        Node* p2 = findSet(n2);
        if(p1 != nullptr && p2 != nullptr){
            if(p1->rank >= p2->rank){
              p1->rank = (p1->rank == p2->rank) ? p1->rank + 1 : p1->rank;
              p2->parent = p1;
            } else p1->parent = p2;
        }
    }

};

int main(){
    int testcases;
    cin>>testcases;
    while(testcases--){
        int n, m;
        cin>>n>>m;
        DisjointSet temp; 

        for(int i = 0; i < n; i++) temp.makeSet(i);

        vector<int> owed(n);

        for(int i = 0; i < n; i++) cin>>owed[i];

        for(int i = 0; i < m;i++){
            int a,b;
            cin>>a>>b;
            temp.unionS(a, b);
        }

        bool isPossible = true;

        unordered_map<int, vector<int>> temp2;

        for(unordered_map<int, Node*>::iterator i = temp.elms.begin(); i != temp.elms.end(); i++) temp2[i->second->parent->data].push_back(i->second->data);

        for(unordered_map<int, vector<int>>::iterator e = temp2.begin(); e != temp2.end(); e++){
            int sum = 0; 
            for(int i = 0; i < (e->second).size();i++){
                sum += owed[(e->second)[i]];
            }
            if(sum != 0){
                isPossible = false; 
                break;
            }
        }

        if(isPossible) cout<<"POSSIBLE"<<endl;
        else cout<<"IMPOSSIBLE"<<endl;

    }
    return 0;

}