不使用upper_bound函数

#include <bits/stdc++.h>
#define int long long
using namespace std;
const int INF = 0x3f3f3f3f;
const int N = 1e5 + 10;
int n , m , cnt , a [N];
signed main()
{
		cin >> n >> m;
		for (int i = 1; i <= n; i++) cin >> a [i];
		while ( m-- )
		{
			cin >> cnt;
            int l = 1 , r = n , ans = 0;
            while (l <= r)
            {
                int mid = l + r >> 1;
                if (cnt >= a [mid])//要查找的值大于当前值
                {
                    if (a [mid] == cnt)
                        ans = max (ans , mid);
                    l = mid + 1;
                } 
                else r = mid - 1;
            }
            if (!ans) cout << "-1\n";
            else cout << ans << "\n";
		}
		return 0;
}

使用upper_bound函数

#include <bits/stdc++.h>
#define int long long
using namespace std;
const int INF = 0x3f3f3f3f;
const int N = 1e5 + 10;
int n , m , cnt , a [N];
signed main()
{
		cin >> n >> m;
		for (int i = 1; i <= n; i++) cin >> a [i];
		while ( m-- )
		{
			cin >> cnt;
            if (a [(upper_bound (a + 1 , a + n + 1 , cnt) - a) - 1] != cnt) cout << "-1\n";
			else cout << (upper_bound (a + 1 , a + n + 1 , cnt) - a) - 1 << "\n" ;
		}
		return 0;
}