P3919 【模板】可持久化线段树 1（可持久化数组）题解 主席树模板题

题目链接：https://www.luogu.com.cn/problem/P3919

解题思路：

主席树纯模板题。

示例程序：

#include <bits/stdc++.h>
using namespace std;
const int maxn = 1000010;
struct Tree {
    int l, r, val;
} tree[maxn*40];
int n, m, root[maxn], tot;

void build(int l, int r, int& rt) {
    rt = ++tot;
    if (l == r) {
        cin >> tree[rt].val;
        return;
    }
    int mid = (l + r) / 2;
    build(l, mid, tree[rt].l);
    build(mid+1, r, tree[rt].r);
}

void update(int p, int val, int l, int r, int pt, int &rt) {
    tree[rt = ++tot] = tree[pt];
    if (l == r) {
        tree[rt].val = val;
        return;
    }
    int mid = (l + r) / 2;
    if (p <= mid) update(p, val, l, mid, tree[pt].l, tree[rt].l);
    else update(p, val, mid+1, r, tree[pt].r, tree[rt].r);
}

int query(int p, int l, int r, int rt) {
    if (l == r) return tree[rt].val;
    int mid = (l + r) / 2;
    if (p <= mid) return query(p, l, mid, tree[rt].l);
    else return query(p, mid+1, r, tree[rt].r);
}

int main() {
    ios::sync_with_stdio(0);
    cin >> n >> m;
    build(1, n, root[0]);
    for (int i = 1; i <= m; i ++) {
        int v, op, p, val;
        cin >> v >> op >> p;
        if (op == 1) {
            cin >> val;
            update(p, val, 1, n, root[v], root[i]);
        }
        else {  // op == 2
            root[i] = root[v];
            cout << query(p, 1, n, root[i]) << endl;
        }
    }
    return 0;
}