POJ 2104 主席树模板题

#include <iostream>
#include <cstdio>
#include <algorithm>
int const maxn = 200010;
using namespace std;
int a[maxn], b[maxn];
//第几个版本的根节点编号
int root[maxn << 5];
int lc[maxn << 5], rc[maxn << 5], sum[maxn << 5];
int sz;//节点个数
int n, m;
int true_n;

void build(int &rt, int l, int r) {
	rt = ++sz;
	if (l == r)	return;
	int mid = (l + r) >> 1;
	build(lc[rt], l, mid);
	build(rc[rt], mid + 1, r);
}

int update(int id, int l, int r, int pos) {
	int _id = ++sz;
	lc[_id] = lc[id], rc[_id] = rc[id], sum[_id] = sum[id] + 1;
	if (l == r)	return _id;
	int mid = (r + l) >> 1;
	if (pos <= mid)
		lc[_id] = update(lc[id], l, mid, pos);
	else
		rc[_id] = update(rc[id], mid + 1, r, pos);
	return _id;
}

int query(int p, int q, int l, int r, int k) {
	if (l == r)	return l;
	int x = sum[lc[q]] - sum[lc[p]];
	int mid = (l + r) >> 1;
	if (x >= k)
		return query(lc[p], lc[q], l, mid, k);
	else
		return query(rc[p], rc[q], mid + 1, r, k - x);
}

int main() {
	while (~scanf("%d %d", &n, &m)) {
		sz = 0;
		for (int i = 1; i <= n; i++) {
			scanf("%d", &a[i]);
			b[i] = a[i];
		}
		sort(b + 1, b + n + 1);
		true_n = unique(b + 1, b + n + 1) - b - 1;
		build(root[0], 1, true_n);
		for (int i = 1; i <= n; i++) {
			int pos = lower_bound(b + 1, b + true_n + 1, a[i]) - b;
			root[i] = update(root[i - 1], 1, true_n, pos);
		}
		while (m--) {
			int l, r, k;
			scanf("%d %d %d", &l, &r, &k);
			cout << b[query(root[l - 1], root[r], 1, true_n, k)] << endl;
		}
	}
	return 0;
}