这道题的本质就是求N叉树的直径！

#include <iostream>
#include <vector>
#include <unordered_map>

using namespace std;
int n, m, p, ans;
unordered_map<int, vector<int>> g;

int dfs(int cur) {
  int first = 0, second = 0;
  for (const int child : g[cur]) {
    int d = dfs(child);
    if (d > first) second = first, first = d;
    else if (d > second) second = d;
  }
  ans = max(ans, first + second);
  return 1 + first;
}

int main(void) {
  scanf("%d%d", &n, &m);
  for (int i = 2; i <= n; ++i) {
    scanf("%d", &p);
    g[p].emplace_back(i);
  }
  for (int i = 1; i <= m; ++i) {
    scanf("%d", &p);
    g[p].emplace_back(-i); // 为了防止和交换机编号重复，我刻意用负数作为电脑的编号！！！
  }
  dfs(1); 
  printf("%d\n", ans);
}