int lca(int a, int b, int parent[])
{
    int ha = 0, hb = 0, x = a, y = b;
    while (parent[a]) ++ha, a = parent[a];
    while (parent[b]) ++hb, b = parent[b];
    while (ha > hb) x = parent[x], --ha;
    while (hb > ha) y = parent[y], --hb;
    while (x != y) x = parent[x], y = parent[y];
    return x;
}