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struct WaveletTree {
using it = vector<ll>::iterator;
WaveletTree *ln = nullptr, *rn = nullptr;
vector<int> b = {0};
ll lo, hi;
WaveletTree(vector<ll> a) : WaveletTree(all(a), // call this
*min_element(all(a)), *max_element(all(a)) + 1) {}
WaveletTree(it from, it to, ll x, ll y) : lo(x), hi(y) {
if (lo + 1 >= hi || from == to) return;
ll mid = (lo + hi) / 2;
auto f = [&](ll x){return x < mid;};
for (it c = from; c != to; c++) {
b.push_back(b.back() + f(*c));
}
it pivot = stable_partition(from, to, f);
ln = new WaveletTree(from, pivot, lo, mid);
rn = new WaveletTree(pivot, to, mid, hi);
}
// kth element in sort[l, r) all 0-indexed
ll kth(int l, int r, int k) {
if (l >= r || k >= r - l) return -1;
if (lo + 1 >= hi) return lo;
int inLeft = b[r] - b[l];
if (k < inLeft) return ln->kth(b[l], b[r], k);
else return rn->kth(l-b[l], r-b[r], k-inLeft);
}
// count elements in[l, r) smaller than k
int countSmaller(int l, int r, ll k) {
if (l >= r || k <= lo) return 0;
if (hi <= k) return r - l;
return ln->countSmaller(b[l], b[r], k) +
rn->countSmaller(l-b[l], r-b[r], k);
}
~WaveletTree() {delete ln; delete rn;}
};
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