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
 */

#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;

int k;

int mod(string s) {
    int r = 0;
    for (int i = 0; i < s.length(); ++i) {
        r *= 10;
        r += s[i] &mdash; '0';
        r %= k;
    }
    return r;
}

int32_t main() {
    fast
    string s;
    cin >> s;
    cin >> k;
    int d;
    cin >> d;
    vector<int> dig(d);
    for (auto &i: dig) {
        cin >> i;
    }
    int p = mod(s);
    if (p == 0) {
        cout << s << endl;
        return 0;
    }
    sort(dig.begin(), dig.end());
    queue<string> q;
    int cnt = 0;
    q.push(s);
    while (!q.empty()) {
        auto t = q.front();
        if (mod(t) == 0) {
            cout << t;
            return 0;
        }
        for (auto i: dig) {
            string c = t;
            c += char(i + 48);
            q.push(c);
        }
        ++cnt;
        if ((cnt + d - 1) / d >= 10000 * d)
            break;
        q.pop();
    }
    cout << -1;
    return 0;
}
/*
 * @Nargulyev
 */

+ It's my birthday today