using Eulerian path

eulerian-path.cpp

#define MAX_V 100001

int id = 0;
int L[MAX_V], R[MAX_V]; // [L, R]
vector<int> adj[MAX_V];

int dfs(int cur) {
    int i = ++id, cnt = 0;
    L[cur] = i;
    for (auto& nxt : adj[cur]) {
        cnt += 1 + dfs(nxt);
    }
    R[cur] = i + cnt;
    return cnt;
}