相比正常查找二分改个条件就行

改个条件就行

#include<bits/stdc++.h>
using namespace std;
int n,a[105],x;
int find(int number)
{
    int l=0,r=n+1;
    while(l+1!=r)
    {
        int mid=l+(r-l)/2;
        if(a[mid]>=number&&a[mid-1]<number) return mid;
        else if(a[mid]>number) r=mid;
        else l=mid;
    }
    return -1;
}
int main()
{
    cin>>n;
    for(int i=1;i<=n;i++)
        cin>>a[i];
    cin>>x;
    cout<<find(x);
}