COGS 2039. 树的统计

#include <cstdio>
#define N 100005

struct Edge
{
    int next,to;
}edge[N];
int ans[N],root,tag[N],n,fa[N],head[N],cnt;
inline int lowbit(int x) {return x&(-x);}
inline int query(int x)
{
    int ans=0;
    for(;x;x-=lowbit(x)) ans+=tag[x];
    return ans;
}
inline void modify(int x)
{
    for(;x<=n;x+=lowbit(x)) tag[x]++;
}
void dfs(int x)
{
    ans[x]=-query(x-1);
    modify(x);
    for(int i=head[x];i;i=edge[i].next)
    {
        int v=edge[i].to;
        if(v!=fa[x])
        dfs(v);
    }
    ans[x]+=query(x-1);
}
int main()
{
    freopen("counttree.in","r",stdin);freopen("counttree.out","w",stdout);
    scanf("%d",&n);
    for(int i=1;i<=n;++i)
    {
        scanf("%d",&fa[i]);
        if(!fa[i]) root=i;
        else
        {
            edge[++cnt].next=head[fa[i]];
            edge[cnt].to=i;
            head[fa[i]]=cnt;
        }
    }
    dfs(root);
    for(int i=1;i<=n;++i) printf("%d ",ans[i]);
    return 0;
}