https://codeforces.com/edu/course/2/lesson/4/4/practice/contest/274684/problem/E

#include<bits/stdc++.h>

#define ll long long

#define fast ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);

using namespace std;

struct seg {
    vector<vector<int>> tree;
    int size;

    void init(int n) {
        size = 1;
        while (size < n)size *= 2;
        for (int i = 0; i < 2 * size; ++i)tree.push_back({0});
    }

    vector<int> combine(vector<int> a, vector<int> b) {
        for (auto i: b)if (i != 0)a.push_back(i);
        sort(a.begin(), a.end());
        return a;
    }

    void set(int i, int v, int x, int lx, int rx) {
        if (rx - lx == 1) {
            tree[x].pop_back();
            tree[x].push_back(v);
            return;
        }
        int mid = (lx + rx) >> 1;
        if (i < mid)set(i, v, 2 * x + 1, lx, mid);
        else set(i, v, 2 * x + 2, mid, rx);
        tree[x] = combine(tree[2 * x + 1], tree[2 * x + 2]);
    }

    void set(int i, int v) {
        set(i, v, 0, 0, size);
    }

    vector<int> get(int l, int r, int x, int lx, int rx) {
        if (l >= rx || lx >= r)return {0};
        if (lx >= l && rx <= r) {
            return tree[x];
        }
        int mid = (lx + rx) >> 1;
        return combine(get(l, r, 2 * x + 1, lx, mid), get(l, r, 2 * x + 2, mid, rx));
    }

    vector<int> get(int l, int r) {
        return get(l, r, 0, 0, size);
    }
};

int32_t main() {
    fast
    int n, q;
    cin >> n >> q;
    seg st;
    st.init(n);
    for (int i = 0; i < n; ++i) {
        st.set(i, 0);
    }
    while (q--) {
        int c;
        cin >> c;
        if (c == 2) {
            int l, r, p;
            cin >> l >> r >> p;
            auto a = st.get(l - 1, r);
            auto ans = upper_bound(a.begin(), a.end(), p) - a.begin();
            cout << ans << endl;
        } else {
            int i, v;
            cin >> i >> v;
            --i;
            st.set(i, v);
        }
    }
    return 0;
}
/*
 * @Nargulyev
 */

link

can someone explain how I can solve this problem?

#include<bits/stdc++.h>

#define ll long long

#define fast ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);

using namespace std;

struct segtree {

    vector<int> tree;
    int size;

    const int zero = 0;

    void init(int n) {
        size = 1;
        while (size < n)size *= 2;
        tree.assign(size * 2 - 1, 0);
    }

    void build(vector<int> &a, int x, int lx, int rx) {
        if (rx - lx == 1) {
            if (lx < a.size())
                tree[x] = a[lx];
        } else {
            int mid = (lx + rx) >> 1;
            build(a, 2 * x + 1, lx, mid);
            build(a, 2 * x + 2, mid, rx);
            tree[x] = tree[2 * x + 1] + tree[2 * x + 2];
        }
    }

    void build(vector<int> &a) {
        init(a.size());
        build(a, 0, 0, size);
    }

    void set(int i, int v, int x, int lx, int rx) {
        if (rx - lx == 1) {
            tree[x] = v;
            return;
        }
        int mid = (lx + rx) >> 1;
        if (i < mid)set(i, v, 2 * x + 1, lx, mid);
        else set(i, v, 2 * x + 2, mid, rx);
        tree[x] = tree[2 * x + 1] + tree[2 * x + 2];
    }

    void set(int i, int v) {
        set(i, v, 0, 0, size);
    }

    int find(int k, int x, int lx, int rx) {
        if (rx - lx == 1) {
            return lx;
        }
        int m = (lx + rx) >> 1;
        if (k < tree[2 * x + 1])return find(k, 2 * x + 1, lx, m);
        else return find(k - tree[2 * x + 1], 2 * x + 2, m, rx);
    }

    int find(int k) {
        find(k, 0, 0, size);
    }
};

int32_t main() {
    fast
    int n, q;
    cin >> n >> q;
    vector<int> a(n);
    for (auto &i: a)cin >> i;
    segtree st;
    st.build(a);
    while (q--) {
        int c;
        cin >> c;
        if (c == 1) {
            int i;
            cin >> i;
            st.set(i, 1 - a[i]);
            a[i] = 1 - a[i];
        } else {
            int k;
            cin >> k;
            cout << st.find(k) << endl;
        }
    }
    return 0;
}
/*
 * @Nargulyev
 */

+ It's my birthday today