Wavlet Tree

#include<bits/stdc++.h>
using namespace std;

const int maxN = 100;
int a[maxN];

struct WavletTree {
    int low,high;
    WavletTree *l, *r;
    vector<int> b;
    WavletTree (int *from,int *to,int x,int y) {
        low = x,high = y;
        if (low==high or from >= to) return;
        int mid = low + (high - low) / 2;
        b.reserve(to - from + 1);
        b.push_back(0);
        auto f = [mid] (int x) {return x <= mid;};
        for (auto it = from; it != to; ++it) b.push_back(b.back() + f(*it));
        auto pivot = stable_partition(from,to,f);
        l = new WavletTree(from,pivot,low,mid);
        r = new WavletTree(pivot,to,mid + 1,high);
    }
    int kth (int l,int r,int k) {
        //kth smallest in range [l,r]
        if (l > r) return 0;
        if (high == low) return low;
        int left = b[r] - b[l - 1];
        int lb = b[l - 1],rb = b[r];
        if (k <= left) return this->l->kth(lb + 1,rb,k);
        return this->r->kth(l - lb,r - rb,k-left);
    }
    int LTE (int l,int r,int k) {
        //Number of elements in subarray A[L...R] that are less than or equal to y.
        if (l > r or k < low) return 0;
        if (high <= k) return r - l + 1;
        int lb = b[l - 1],rb = b[r];
        return this->l->LTE(lb + 1,rb,k) + this->r->LTE(l - lb,r - rb,k);
    }
    int count (int l,int r,int k) {
        //Number of occurrences of element x in subarray A[L...R].
        if (l > r or k < low or k > high) return 0;
        if (low == high) return r - l + 1;
        int lb = b[l - 1],rb = b[r];
        int mid = low + (high - low) / 2;
        if (k <= mid) return this->l->count(lb + 1,rb,k);
        return this->r->count(l - lb,r - rb,k);
    }
    ~WavletTree () {delete l; delete r;};
};

int main () {
     ios::sync_with_stdio(false);
     cin.tie(nullptr);
     cout.tie(nullptr);
     int T = 1;
     //~ cin >> T;
     for (int test_case = 1; test_case <= T; ++test_case) {
         int n;
         cin >> n;
         for (int i = 1; i <= n; ++i) cin >> a[i];
         WavletTree T(a + 1,a + n + 1,1,10000);
         cout << T.LTE(2,6,4) << '\n';
     }
     //cerr << "Time elapsed :" << clock() * 1000.0 / CLOCKS_PER_SEC << " ms" << '\n';
}