三代目老师教的

#include<bits/stdc++.h>
using namespace std;
int n,mid,l,r,sum,a[1000010],x;
int main(){
cin>>n;
for(int i=1;i<=n;i+=1){
cin>>a[i];
}
cin>>x;
l=1;
r=n;
while(l<=r){
mid=(l+r)/2;
if(a[mid]>x){
r=mid-1;
}
else if(a[mid]<x){
l=mid+1;
}
else{
cout<<mid;
return 0;
}
}
cout<<-1;
}