二分查找，二分上界，二分下界

#include<iostream>
#include<algorithm>
#include<cstdio>
using namespace std;
const int masn=1e6+10;
int x[masn],y[masn];
//区间左闭右开，从0或1开始都可以
int binary_sorce(int t,int l,int r) {
    int mid;
    while(l<r) {
        mid=(l+r)>>1;
        if(x[mid]>=t) r=mid;
        else l=mid+1;
    }
    return x[l]==t?l:-1;
}
int lower_binary(int t,int l,int r) {
    int mid;
    while(l<r) {
        mid=(l+r)>>1;
        if(x[mid]>=t) r=mid;
        else l=mid+1;
    }
    return l;
}
int upper_binary(int t,int l,int r) {
    int mid;
    while(l<r) {
        mid=(l+r)>>1;
        if(t>=x[mid]) l=mid+1;
        else r=mid;
    }
    return l;
}
int main() {
    int num,n;
    while(~scanf("%d",&num)) {
        for(int i=0; i<num; i++)
            scanf("%d",&x[i]);
        sort(x,x+num);
        while(~scanf("%d",&n)) {
            printf("上界：%d\n",upper_binary(n,0,num));
            printf("下界：%d\n",lower_binary(n,0,num));
            printf("是否存在：%d\n",binary_sorce(n,0,num));
        }
    }
    return 0;
}