可持久化线段树，即主席树。

每次修改的时候不修改原来的节点，暴力建新节点，充分运用了函数式编程的思想。

模板题：给定一个数列，\(m\) 次询问求区间 \([l,r]\) 内的第 \(k\) 大。

利用前缀和思想：

#include 
    
     using namespace std;const int MAXN = 2e5 + 5;struct node {    int val;    node *lchild, *rchild;} *rt[MAXN];int a[MAXN], subA[MAXN], n, m, cnt = 0;node *newNode(int val, node *lc, node *rc) {    node *ptr = new node;    ptr->lchild = lc; ptr->rchild = rc; ptr->val = val;    return ptr;}void build(node *&cur, int l, int r) {    if(l < r) {        cur = newNode(0, NULL, NULL);        int mid = (l + r) >> 1;        build(cur->lchild, l, mid);        build(cur->rchild, mid + 1, r);    } else cur = newNode(0, NULL, NULL);}void modify(node *&cur, node *fa, int l, int r, int x) {    cur = newNode(fa->val + 1, fa->lchild, fa->rchild);    if(l != r) {        int mid = (l + r) >> 1;        if(x <= mid) modify(cur->lchild, cur->lchild, l, mid, x);        else modify(cur->rchild, cur->rchild, mid + 1, r, x);    }}int query(node *u, node *v, int l, int r, int k) {    if(l == r) return l;    int mid = (l + r) >> 1, lessSize = v->lchild->val - u->lchild->val;    if(lessSize >= k)        return query(u->lchild, v->lchild, l, mid, k);    else return query(u->rchild, v->rchild, mid + 1, r, k - lessSize);}int main() {    scanf("%d%d", &n, &m);    for(int i = 0; i < n; i++) {        scanf("%d", a + i);        subA[i] = a[i];    }    sort(subA, subA + n);    int size = unique(subA, subA + n) - subA;    for(int i = 0; i < n; i++)        a[i] = lower_bound(subA, subA + size, a[i]) - subA + 1;    build(rt[cnt++], 1, n);    for(int i = 0; i < n; i++) {        modify(rt[cnt], rt[cnt - 1], 1, n, a[i]);        cnt++;    }    while(m--) {        int x, y, k;        scanf("%d%d%d", &x, &y, &k);        printf("%d\n", subA[query(rt[x - 1], rt[y], 1, n, k) - 1]);    }    return 0;}