P3919 【模板】可持久化线段树 1（可持久化数组）（主席树）

这道题要注意空间不能开太小，还有我第一次是build函数忘写return了，我很奇怪，结构体数组开小后，有一个报错和tle，不知道tle是怎么来的

#include<iostream>
#include<set>
#include<map>
#include<algorithm>
#include<vector>
#include<cmath>
#include<climits>
#include<cstring>
#define int long long
const int N = 1e6+5;
using namespace std;
char* p1, * p2, buf[100000];
#define nc() (p1==p2 && (p2=(p1=buf)+fread(buf,1,100000,stdin),p1==p2)?EOF:*p1++)
int read()
{
	int x = 0, f = 1;
	char ch = nc();
	while (ch < 48 || ch>57)
	{
		if (ch == '-')
			f = -1;
		ch = nc();
	}
	while (ch >= 48 && ch <= 57)
		x = x * 10 + ch - 48, ch = nc();
	return x * f;
}
int a[N], id=0,root[N];
struct tree {
	int left;
	int right;
	int val;
}tr[N*40];
void build(int& x, int l, int r) {
	x = ++id;
	if (l == r) { tr[x].val = a[l]; return; }
	int mid = (l + r) >> 1;
	build(tr[x].left, l, mid);
	build(tr[x].right, mid + 1, r);
}
void change(int x, int& y, int l, int r,int p ,int k) {
	y = ++id;
	tr[y] = tr[x];
	if (l == r) { tr[y].val = k; return; }
	int mid = (l + r) >> 1;
	if (p <= mid)change(tr[x].left, tr[y].left, l, mid, p, k);
	else change(tr[x].right, tr[y].right, mid+1, r, p, k);
}
int query(int x,int l, int r, int p) {
	if (l == r) { return tr[x].val; }
	int mid = (l + r) >> 1;
	if (p <= mid)return query(tr[x].left, l, mid, p);
	else return query(tr[x].right, mid+1, r, p);
}
signed main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);
	int n, m;
	cin >> n >> m;
	for (int i = 1; i <= n; i++)cin >> a[i];
	build(root[0], 1, n);
	for (int i = 1; i <= m; i++) {
		int ver, op;
		cin >> ver >> op;
		if (op == 1) {
			int p, k;
			cin >> p >> k;
			change(root[ver], root[i], 1, n, p, k);
		}
		else {
			int p;
			cin >> p;
			cout << query(root[ver], 1, n, p) << endl;
			root[i] = root[ver];
		}
	}
	return 0;
}