这才是最好的模板！

int l = 0, r = a.size() - 1;

while (l <= r) {
    int mid = l + (r - l) / 2;
    if (a[mid] >= k) r = mid - 1;
    else l = mid + 1;
}
while (l <= r) {
    int mid = l + (r - l) / 2;
    if (a[mid] <= k) l = mid + 1;
    else r = mid - 1;
}

代码

#include <iostream>
#include <string>
#include <algorithm>
#include <cstdio>
#include <vector>
#define ios_sync ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
using namespace std;

const int N = 100010;
vector<int> a;

int main() {
    ios_sync;
    int n, q;
    cin >> n >> q;
    for (int i = 0; i < n; i ++) {
        int x;
        cin >> x;
        a.push_back(x);
    }
    sort(a.begin(), a.end());

    for (int i = 0; i < q; i ++) {
        int k;
        cin >> k;

        int l = 0, r = a.size() - 1;

        while (l <= r) {
            int mid = l + (r - l) / 2;
            if (a[mid] >= k) r = mid - 1;
            else l = mid + 1;
        }
        int x;
        if (a[l] != k) x = -1;
        else x = l;

        l = 0, r = a.size() - 1;

        while (l <= r) {
            int mid = l + (r - l) / 2;
            if (a[mid] <= k) l = mid + 1;
            else r = mid - 1;
        }
        int y;
        if (a[r] != k) y = -1;
        else y = r;

        cout << x << ' ' << y << '\n';
    }
    return 0;
}