fhqTreap查不了前驱求助

//P6136
#include <bits/stdc++.h>
using namespace std;

inline int read(){
    int s = 0; char ch = getchar();
    while(ch < '0' || ch > '9') ch = getchar();
    while(ch >= '0' && ch <= '9') s = s * 10 + ch - '0', ch = getchar();
    return s;
}

const int N = 1100010;
struct fhqTreap{
    int ch[N][2], val[N], pri[N], siz[N], tot;
    int root, x, y, z;
    fhqTreap(){
        srand(time(NULL));
        memset(ch, 0, sizeof(ch));
        memset(val, 0, sizeof(val));
        memset(pri, 0, sizeof(pri));
        memset(siz, 0, sizeof(siz));
        tot = root = x = y = z = 0;
        newnode(0xcfcfcfcf), newnode(0x3f3f3f3f);
    }
    void update(int x){ siz[x] = siz[ch[x][0]] + siz[ch[x][1]] + 1; }
    int newnode(int v){ siz[++tot] = 1, val[tot] = v, pri[tot] = rand(); return tot;}
    int merge(int x, int y){
        if(!x || !y) return x + y;
        if(pri[x] < pri[y]){ ch[x][1] = merge(ch[x][1], y), update(x); return x; }
        else               { ch[y][0] = merge(x, ch[y][0]), update(y); return y; }
    }
    void split(int now, int k, int &x, int &y){
        if(!now){ x = y = 0; return; }
        if(val[now] <= k) x = now, split(ch[now][1], k, ch[now][1], y);
        else              y = now, split(ch[now][0], k, x, ch[now][0]);
        update(now);
    }
    int kth(int now, int k){
        while(true){
            if(k <= siz[ch[now][0]]) now = ch[now][0];
            else if(k == siz[ch[now][0]] + 1) return now;
            else k -= siz[ch[now][0]] + 1, now = ch[now][1];
        }
    }
    void Insert(int k){
        split(root, k, x, y);
        root = merge(merge(x, newnode(k)), y);
    }
    void Delete(int k){
        split(root, k, x, z);
        split(x, k-1, x, y);
        y = merge(ch[y][0], ch[y][1]);
        root = merge(merge(x, y), z);
    }
    int GetRankByVal(int k){
        split(root, k-1, x, y);
        int ans = siz[x] + 1;
        root = merge(x, y);
        return ans;
    }
    int GetValByRank(int k){
        return val[kth(root, k)];
    }
    int GetPre(int k){
        split(root, k-1, x, y);
        int ans = val[kth(x, siz[x])];
        root = merge(x, y);
        return ans;
    }
    int GetNxt(int k){
        split(root, k, x, y);
        int ans = val[kth(y, 1)];
        root = merge(x, y);
        return ans;
    }
} T;

int main(){
    int n, m, last = 0, ans = 0, op, x;
    n = read(), m = read();
    for(int i = 1; i <= n; ++ i) T.val[i] = read();
    for(int i = 1; i <= m; ++ i){
        op = read(), x = read() ^ last;
        switch(op){
            case 1: T.Insert(x); break;
            case 2: T.Delete(x); break;
            case 3: last = T.GetRankByVal(x); break;
            case 4: last = T.GetValByRank(x); break;
            case 5: last = T.GetPre(x); break;
            case 6: last = T.GetNxt(x); break;
        }
        if(op >= 3) ans ^= last;
    }
    printf("%d\n", ans);
    return 0;
}

fhqTreap板子能过P3369