rt，TLE#11，代码如下

#include <iostream>
#include <cstdio>
const int BUFFER_SIZE = 1 << 20;
char rb[BUFFER_SIZE], *rp = rb, *rt = rb;
inline char read_char() {
    return rp == rt ? (rt = rb + fread(rb, 1, BUFFER_SIZE, stdin), rp = rb, *rp++) : *rp++;
}
inline int read_int() {
    int x = 0;
    char ch = read_char(), flag = 0;
    while (ch != '-' && (ch < '0' || ch > '9')) {
        ch = read_char();
    }
    if (ch == '-') {
        flag = 1;
        ch = read_char();
    }
    for (x = 0; ch >= '0' && ch <= '9'; ch = read_char()) {
        x = x * 10 + (ch - '0');
    }
    return flag ? -x : x;
}
int p1 = -1;
char buf[BUFFER_SIZE];
const int p2 = BUFFER_SIZE - 1;
inline void flush() {
    fwrite(buf, 1, p1 + 1, stdout);
    p1 = -1;
}
inline void put_char(char s) {
    if (p1 == p2)
        flush();
    buf[++p1] = s;
}
inline void write_int(int n) {
    static char s[40];
    static int top = 0;
    static int flag = n < 0;
    n = n > 0 ? n : -n;
    while (s[++top] = (n % 10) + '0', n /= 10);
    if (flag) s[++top] = '-';
    while (put_char(s[top]), --top);
    return;
}
const int nn = 1e6 + 5;
int a[nn], n, m;
namespace SegTree {
    struct node {
        int val;
        node *left_son, *right_son;
        node() {
            val = 0;
            left_son = right_son = NULL;
        }
    };
    node *root[nn];
    void build(node *p, const int l, const int r) {
        if (l == r) {
            p -> val = a[l];
            return;
        }
        p -> left_son = new node();
        p -> right_son = new node();
        int mid = l + r >> 1;
        build(p -> left_son, l, mid);
        build(p -> right_son, mid + 1, r);
        return;
    }
    node* modify(node *p, const int &k, const int &val, const int l, const int r) {
        node *now = new node();
        *now = *p;
        p = now;
        if (l == r) {
            p -> val = val;
            return p;
        }
        int mid = l + r >> 1;
        if (k <= mid) p -> left_son = modify(p -> left_son, k, val, l, mid);
        else p -> right_son = modify(p -> right_son, k, val, mid + 1, r);
        return p;
    }
    int ask(node *p, const int &k, const int l, const int r) {
        if (l == r) return p -> val;
        int mid = l + r >> 1;
        if (k <= mid) return ask(p -> left_son, k, l, mid);
        else return ask(p -> right_son, k, mid + 1, r);
    }
};
using namespace SegTree;
int main() {
    n = read_int();
    m = read_int();
    for (register int i = 1; i <= n; ++i) a[i] = read_int();
    root[0] = new node();
    build(root[0], 1, n);
    int ver, opt, k, val;
    for (register int i = 1; i <= m; ++i) {
        ver = read_int();
        opt = read_int();
        k = read_int();
        if (opt == 1) {
            val = read_int();
            root[i] = modify(root[ver], k, val, 1, n);
        } else {
            printf("%d\n", ask(root[ver], k, 1, n));
            root[i] = root[ver];
        }
    }
}

感觉不像常数问题，是哪里写假了吗