代码

#include "cstdio"

using namespace std;

#define Max(a, b) (a) = (b) > (a) ? (b) : (a)

const int N = 10000;

int degOut[N+1];
int father[N+1];
int height[N+1];
int n, m, v, k, ans;

bool topo(int i)
{
    --degOut[i];                // lock
    --degOut[father[i]];        // topologic
    Max(ans, height[i] + height[father[i]] + 1);
    Max(height[father[i]], height[i]+1);

    if(--k == 1) return printf("%d", ans) && 0;
    if(!degOut[father[i]]) return topo(father[i]);
}

int main()
{
    scanf("%d %d", &n, &m);
    for(int i=2; i<=n; ++i) scanf("%d", &father[i]), ++degOut[father[i]];
    // ++degOut[v], // the deg of PCs should be excluded
    for(int i=1; i<=m; ++i) scanf("%d", &v), height[v] = 1;

    k = n;
    for(int i=1; i<=n; ++i)
        if(!degOut[i])
            topo(i);
}


void print(int i, int k)
{
    printf(">>> i: %d, k: %d->%d", i, k, k-1);
    printf("\nheights: ");
    for(int i=1; i<=n; ++i) printf("%2d ", height[i]);
    printf("\ndegOuts: ");
    for(int i=1; i<=n; ++i) printf("%2d ", degOut[i]);
    printf("\n\n");
}