树状数组 || POJ 3321 Apple Tree

一道dfs序+树状数组的题

因为并没有get到dfs序以及对树状数组也不熟练卡了很久orz

dfs序：

in和out是时间戳

dfs序可以将树转化成为一个序列，满足区间 -> 子树

然后就可以用树状数组之类的维护序列的东东来维护了

int idx = 0;
void dfs(int u, int fa)
{
    seq[++idx] = u;
    in[u] = idx;
    for(int i = head[u];i;i = nxt[i])
    {
        int v = l[i];
        if(v != fa)
        {
            dfs(v, u);
        }
    }
    out[u] = idx;
}

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <queue>
using namespace std;
const int SZ = 200010;
const int INF = 1e9+10;
int head[SZ], nxt[SZ], l[SZ], tot = 0;
int in[SZ], out[SZ], seq[SZ], a[SZ];
void build(int f, int t)
{
    l[++tot] = t;
    nxt[tot] = head[f];
    head[f] = tot;
}
int idx = 0;
void dfs(int u, int fa)
{
    seq[++idx] = u;
    in[u] = idx;
    for(int i = head[u];i;i = nxt[i])
    {
        int v = l[i];
        if(v != fa)
        {
            dfs(v, u);
        }
    }
    out[u] = idx;
}
int d[SZ], n;
void add(int i, int x)
{
    for(; i <= n; i += (i & -i))
        d[i] += x;
}
int get(int i)
{
    int ans = 0;
    for(;i;i -= (i & -i))
        ans += d[i];
    return ans;
}
int query(int l, int r)
{
    return get(r) - get(l - 1);
}
int main()
{
    scanf("%d", &n);
    for(int i = 0; i < n-1; i++)
    {
        int x, y;
        scanf("%d %d", &x, &y);
        build(x, y);
        build(y, x);
    }
    dfs(1, 0);
    /*for(int i = 0; i < 11; i++) printf("%d ", in[i]); printf("
");
    for(int i = 0; i < 11; i++) printf("%d ", out[i]); printf("
");
    for(int i = 0; i < 25; i++) printf("%d ", seq[i]); printf("
");*/
    int m;
    scanf("%d", &m);
    for(int i = 1; i <= n; i++)
        add(i, 1), a[i] = 1;
    while(m--)
    {
        char op;
        int x;
        scanf("%c", &op);
        scanf("%c", &op);
        scanf("%d", &x);
        if(op == 'Q')
            printf("%d
", query(in[x], out[x]));
        if(op == 'C')
        {
            if(a[in[x]]) {a[in[x]] = 0; add(in[x], -1); }
            else {a[in[x]] = 1; add(in[x], 1); }
        }
    }
    return 0;
}