CODE WITH ANNOTATION

代码中 $-6$ 表示左括号’[’,用 $-9$ 表示右括号’]’.
在这里插入图片描述 emmmm…
#include<bits/stdc++.h>
using namespace std;
char cb[1<<15],*cs=cb,*ct=cb;
#define getc() (cs==ct&&(ct=(cs=cb)+fread(cb,1,1<<15,stdin),cs==ct)?0:*cs++)
template<class T>inline void read(T &res) {
    char ch; int flg = 1; for(;!isdigit(ch=getc());)if(ch=='-')flg=-flg;
    for(res=ch-'0';isdigit(ch=getc());res=res*10+ch-'0'); res*=flg;
}
typedef long long LL;

const int MAXN = 100005;
const int MAXV = 300005;
const int INF = 0x3f3f3f3f;
int n, q, tot, black, dfn[MAXN], fir[MAXN], cnt, seq[MAXV]; bool col[MAXN];
struct edge { int to, nxt; }e[MAXN<<1];
inline void add(int u, int v) {
    e[++cnt] = (edge) { v, fir[u] }, fir[u] = cnt;
    e[++cnt] = (edge) { u, fir[v] }, fir[v] = cnt;
}
void dfs(int u, int ff)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           {
    seq[++tot] = -6; //[
    seq[dfn[u]=++tot] = u;
    for(int i = fir[u]; i; i = e[i].nxt)
        if(e[i].to != ff) dfs(e[i].to, u);
    seq[++tot] = -9; //]
}
inline void chkmax(int &x, int y) { if(y > x) x = y; };
inline int max(int x, int y, int z) { return max(max(x, y), z); }
struct seg { //维护形如"]]]]][["的串
    int L, R, dis; //L:cnt(']'), R:cnt(']')
    int suf_plus, suf_minus; //suffix : max(L + R), max(L - R) 前缀最大值
    int pre_plus, pre_minus; //prefix : max(R + L), max(R - L) 后缀最大值
    inline void init(int i) {
        dis = -INF;
        L = (seq[i] == -9); //']'
        R = (seq[i] == -6); //'['
        if(seq[i] > 0 && col[seq[i]]) suf_plus = suf_minus = pre_plus = pre_minus = 0;
        else suf_plus = suf_minus = pre_plus = pre_minus = -INF;
    }
    inline friend seg operator +(const seg ls, const seg rs) {
        //MAIN IDEA: use "max()" to avoid category talk(用各种姿势的max避免分类讨论)
        int a = ls.L, b = ls.R, c = rs.L, d = rs.R; seg re;
    
        re.dis = max(ls.dis, rs.dis);
        chkmax(re.dis, ls.suf_plus + rs.pre_minus);
        chkmax(re.dis, ls.suf_minus + rs.pre_plus);
        
        // a]  <->  [d (a and d are symmetrical(对称))
        // [b  <->  c] (b and c are symmetrical)
        re.suf_plus = max(ls.suf_plus - c + d, ls.suf_minus + c + d, rs.suf_plus);
		//前两个位置取max就避免了分类讨论,本来是要分(1.b>=c 2.b<c)两种情况考虑的.
		//这样取max巧妙地避免写很多if(但也很有可能写爆)
        re.pre_plus = max(rs.pre_plus - b + a, rs.pre_minus + b + a, ls.pre_plus);
        
        re.suf_minus = max(ls.suf_minus + c - d, rs.suf_minus);
        re.pre_minus = max(rs.pre_minus + b - a, ls.pre_minus);
        
        if(b >= c) re.L = a, re.R = d + b-c;
        else re.L = a + c-b, re.R = d;
        
        return re;
    }
}t[MAXV<<2];
inline void upd(int i) { t[i] = t[i<<1] + t[i<<1|1]; } //简洁的update 2333
void build(int i, int l, int r) {
    if(l == r) { t[i].init(l); return; }
    int mid = (l + r) >> 1;
    build(i<<1, l, mid);
    build(i<<1|1, mid+1, r);
    upd(i);
}
void modify(int i, int l, int r, int x) {
    if(l == r) { t[i].init(l); return; }
    int mid = (l + r) >> 1;
    if(x <= mid) modify(i<<1, l, mid, x);
    else modify(i<<1|1, mid+1, r, x);
    upd(i);
}
int main() {
    read(n);
    for(int x, y, i = 1; i < n; ++i)
        read(x), read(y), add(x, y);
    dfs(1, 0);
    for(int i = 1; i <= n; ++i) col[i] = 1, ++black;
    build(1, 1, tot);
    read(q); char s; int x;
    while(q--) {
        while(!isalpha(s=getc()));
        if(s == 'G') {
            if(!black) puts("-1");
            else if(black == 1) puts("0");
            else printf("%d
", t[1].dis);
        }
        else {
            read(x);
            if(!col[x]) ++black;
            else --black;
            col[x] ^= 1;
            modify(1, 1, tot, dfn[x]);
        }
    }
}