用 iota(fa + 1, fa + 1 + n, 1); 初始化并查集的 fa 数组，WA

而 for 循环赋值就AC了

为什么?

mst ->

inline ll solve() {
    ll cnt = 0, ans = 0;
    sort(s + 1, s + 1 + tot);
    rep(i, 1, n)fa[i] = i;
  //iota(fa + 1, fa + 1 + n, 1);
    for (const auto &x: s) {
        ll u = find(x.u), v = find(x.v);
        if (u == v)continue;
        fa[v] = u, cnt++, ans += x.w;
        if (cnt == n - 1)return ans;
    }
    return -1;
}