题目链接： https://www.luogu.org/problem/P3834
首先要离散化，然后主席树模板。
1 #include<cstdio>
2 #include<cstring>
3 #include<algorithm>
4 #define mid (l+r)/2
5 using namespace std;
6
7 const int N = 200010;
8 int n, q, m, cnt = 0;
9 int a[N], b[N], T[N];
10 int sum[N<<5], L[N<<5], R[N<<5];
11
12 inline int build(int l, int r)
13 {
14 int rt = ++ cnt;
15 sum[rt] = 0;
16 if (l < r){
17 L[rt] = build(l, mid);
18 R[rt] = build(mid+1, r);
19 }
20 return rt;
21 }
22
23 inline int update(int pre, int l, int r, int x)
24 {
25 int rt = ++ cnt;
26 L[rt] = L[pre]; R[rt] = R[pre]; sum[rt] = sum[pre]+1;
27 if (l < r){
28 if (x <= mid) L[rt] = update(L[pre], l, mid, x);
29 else R[rt] = update(R[pre], mid+1, r, x);
30 }
31 return rt;
32 }
33
34 inline int query(int u, int v, int l, int r, int k)
35 {
36 if (l >= r) return l;
37 int x = sum[L[v]] - sum[L[u]];
38 if (x >= k) return query(L[u], L[v], l, mid, k);
39 else return query(R[u], R[v], mid+1, r, k-x);
40 }
41
42 int main()
43 {
44 scanf("%d%d", &n, &q);
45 for (int i = 1; i <= n; i ++){
46 scanf("%d", &a[i]);
47 b[i] = a[i];
48 }
49 sort(b+1, b+1+n);
50 m = unique(b+1, b+1+n)-b-1;
51 T[0] = build(1, m);
52 for (int i = 1; i <= n; i ++){
53 int t = lower_bound(b+1, b+1+m, a[i])-b;
54 T[i] = update(T[i-1], 1, m, t);
55 }
56 while (q --){
57 int x, y, z;
58 scanf("%d%d%d", &x, &y, &z);
59 int t = query(T[x-1], T[y], 1, m, z);
60 printf("%d\n", b[t]);
61 }
62 return 0;
63 }