Tree Reconstruction - codeforce

int n;
int use[1005];

P ans[1004];

int main()
{
    cin >> n;
    vector<int> a;
    rep(i, 1, n) {
        int u, v;
        cin >> u >> v;
        if (!use[u]) a.pb(u);
        use[u]++;
        use[v]++;
    }
    sort(Range(a));
    if (use[n] != n - 1) {
        puts("NO");
    }
    else {
        int cnt = 0;
        bool flag = false;
        for (auto v : a) {
            int t = use[v] - 1, tt = 0, last = n;
            rep(i, 1, v) if (!use[i] && tt < t) {
                tt++;
                use[i] = 1;
                ans[++cnt] = P(last, i);
                last = i;
            }
            ans[++cnt] = P(last, v);
            if (tt != t) {
                flag = true;
                break;
            }
        }
        if (flag) {
            puts("NO");
        }
        else {
            puts("YES");
            rep(i, 1, n) cout << ans[i].first << " " << ans[i].second << endl;
        }
    }
    return 0;
}