线段树合并与分裂

#include <bits/stdc++.h>
#define ll long long
using namespace std;

const int maxn = 1e5 + 10;

int a[maxn], b[maxn];
int lson[maxn], rson[maxn];
int tree[maxn];
int root[maxn];
int cnt = 0;
int n;
int ans[maxn];
vector<int>G[maxn];
int temp;

int newnode()
{
    ++cnt;
    lson[cnt]=rson[cnt]=0;
    return cnt;
}

void push_up(int x)
{
    tree[x]=tree[lson[x]]+tree[rson[x]];
}

int build(int x, int l ,int r,int num)
{
    if(!x)
        x=newnode();
    if(l==r)
    {
        tree[x]=1;
        return x;
    }
    int mid = (l+r)/2;
    if(num<=mid) lson[x] = build(lson[x], l, mid, num);
    else   rson[x] = build(rson[x], mid+1, r, num);
    push_up(x);
    return x;
}

int query(int x, int l, int r, int ql, int qr)
{
    if(ql<=l && qr>=r)
        return tree[x];
    int mid = (l+r)/2;
    int ans = 0;
    if(ql<=mid && lson[x]) ans+=query(lson[x], l, mid, ql, qr);
    if(qr>mid && rson[x])  ans+=query(rson[x], mid+1,r, ql,qr);
    return ans;
}

int mercy(int root1, int root2)
{
    if(!root1) return root2;
    if(!root2) return root1;
    lson[root1] = mercy(lson[root1], lson[root2]);
    rson[root1] = mercy(rson[root1], rson[root2]);
    tree[root1] += tree[root2];
    return root1;
}

void dfs(int now, int fa)
{
    int len = G[now].size();
    for(int i = 0; i < len; i++)
    {
        int k = G[now][i];
        if(k == fa)
            continue;
        dfs(k, now);
        ans[now]+=query(root[k],1,temp,min(a[now]+1,temp),temp);
        mercy(root[now], root[k]);
    }
}

int main()
{
    scanf("%d", &n);
    for(int i = 1; i <= n; i++)
    {
        scanf("%d", &a[i]);
        b[i]=a[i];
    }
    sort(b+1, b+n+1);
    temp = unique(b+1, b+n+1)-b-1;
    for(int i = 1; i <=n; i++)
    {
        a[i] = lower_bound(b+1, b+temp+1,a[i])-b;
    }
    int x;
    for(int i = 2; i <= n;i++)
    {
        scanf("%d", &x);
        G[i].push_back(x);
        G[x].push_back(i);
    }
    for(int i = 1; i <= n; i++)
    {
        root[i] = build(root[i], 1 , temp, a[i]);
    }
    dfs(1, 1);
    for(int i = 1; i <= n; i++)
    printf("%d ", ans[i]);
    return 0;
}