lower_bound最简单解法

灵机一动

#include<bits/stdc++.h>
#include<algorithm>
using namespace std;
long long n, s[110], x, ans;
int main(){
    cin>>n;
    for (int i=1; i<=n; i++){
        cin>>s[i];
    }
    cin>>x;
    ans = lower_bound(s+1, s+n+1, x) - s;
    if (s[ans]!=x){
        cout<<"-1";
    } else{
        cout<<ans;
    }
    return 0;
}