题解 | 【模板】二分

二分模板，按照题意模拟即可，记得讨论清楚边界

#include <bits/stdc++.h>
using namespace std;
#define int long long
const int N = 2e5 + 5;
int __t = 1, n, q, op, l, r, x, k;
void solve() {
    vector<int> a;
    cin >> n >> q;
    for (int i = 0; i < n; i++)
        cin >> x, a.push_back(x);
    while (q--) {
        cin >> op >> l >> r >> x;
        if (op == 1)
            k = lower_bound(a.begin() + l, a.begin() + r, x) - a.begin();
        if (op == 2)
            k = upper_bound(a.begin() + l, a.begin() + r, x) - a.begin();
        if (op == 3)
            k = lower_bound(a.begin() + l, a.begin() + r, x) - a.begin() - 1;
        if (op == 4)
            k = upper_bound(a.begin() + l, a.begin() + r, x) - a.begin() - 1;
        cout << ((k < r && k >= l) ? a[k] : -1) << "\n";
    }
}

int32_t main() {
#ifdef ONLINE_JUDGE
    ios::sync_with_stdio(false);
    cin.tie(0);
#endif
    // cin >> __t;
    while (__t--)
        solve();
    return 0;
}