Prufer Code from Tree

//Trần Việt Anh - 8/11/2019 - 11:15PM
#include<iostream>
#include<map>
#include<vector>
#include <queue>
#include <set>

using namespace std;

int main() {
    int lineCount;
    priority_queue<int> vertex;
    map<int, vector<int> > tree;
    map<int, int> vertexCount;
    set<int> nodeSet;

    cin >> lineCount;
    for (int i = 0; i < lineCount; i++) {
        int x, y; cin >> x >> y;
        nodeSet.insert(x); nodeSet.insert(y);

        map<int, int>::iterator it = vertexCount.find(x);

        if (it == vertexCount.end()) {
            //cout << "new " << x << endl;
            vertex.push(-x);
            vertexCount[x] = 1;
            vector<int> values(1, y);
            tree[x] = values;
        }
        else {
            vertexCount[x] += 1;
            tree[x].push_back(y);
        }

        it = vertexCount.find(y);
        if (it == vertexCount.end()) {
            //cout << "new " << y << endl;
            vertex.push(-y);
            vertexCount[y] = 1;
            vector<int> values(1, x);
            tree[y] = values;
        }
        else {
            vertexCount[y] += 1;
            tree[y].push_back(x);
        }
    }

    int n = nodeSet.size();
    vector<int> sortedVertex(0);
    while (vertex.size() != 0) {
        sortedVertex.push_back(-vertex.top());
        vertex.pop();
    }

    // for (int i = 0; i < sortedVertex.size(); i++) {
    //  cout << sortedVertex[i] << " ";
    // }
    // cout << endl;

    // for (int i = 0; i < sortedVertex.size(); i++) {
    //  cout << "Node Value: " << sortedVertex[i] << endl;
    //  cout << "Count: " << vertexCount[sortedVertex[i]] << endl;
    //  cout << "Size: " << tree[sortedVertex[i]].size() << endl;
    //  for (int j = 0; j < tree[sortedVertex[i]].size(); j++)
    //      cout << tree[sortedVertex[i]][j] << "-";
    //  cout << endl;
    // }

    bool checked[sortedVertex.size()];
    for (int i = 0; i < sortedVertex.size(); i++)
        checked[i] = false;

    for (int i = 0; i < n - 2; i++) {
        for (int nodeIndex = 0; nodeIndex < sortedVertex.size(); nodeIndex++) {
            if (!checked[nodeIndex]) {
                checked[nodeIndex] = true;
                bool valid = true;
                for (int j = 0; j < tree[sortedVertex[nodeIndex]].size(); j++)
                    if (vertexCount[tree[sortedVertex[nodeIndex]][j]] == 1) {
                        valid = false;
                        break;
                    }
                if (valid) {
                    int connectedVertex = *tree[sortedVertex[nodeIndex]].begin();
                    cout << connectedVertex << " ";
                    vertexCount[connectedVertex]--;
                    for (int ind = 0; ind < tree[connectedVertex].size(); ind ++)
                        if (tree[connectedVertex][ind] == sortedVertex[nodeIndex]) {
                            tree[connectedVertex].erase(tree[connectedVertex].begin() + ind);
                            break;
                        }
                }
            }
        }
    }

    return 0;
}