题解看了，没懂

int calc(int v, int k) {
  int tot = 0, mx = -1;
  for (int u : g[v]) {
    tot += sz[u];
    if (mx == -1 || sz[mx] < sz[u]) mx = u;
  }
  if (tot == 0) return 0;
  if (sz[mx] - k <= tot - sz[mx])
    return (tot - k) / 2;
  int add = tot - sz[mx];
  return add + calc(mx, max(0, k + add - 1));
}