时间超限的问题

为什么这个程序在有些网站运行出来说时间超限？？？

#include <stdio.h>
#include <stdlib.h>
long long unsigned bn[100000000];
int findBKNum(int start,int end,long long unsigned bkNum)
{
    int mid;
    while(start<=end){
        mid=(start+end)/2;
        if(bkNum==bn[mid])
            return mid;
        if(bkNum>bn[mid])
            start=mid+1;
        else
            end=mid-1;
    }
    return -1;

}
int main()
{
    int n,i,k;
    long long unsigned bkNum;
    scanf("%d",&n);
    for(i=0;i<n;i++)
    scanf("%llu",&bn[i]);
    scanf("%d",&k);
    while(k--){
        scanf("%llu",&bkNum);
        printf("%d\n",findBKNum(0,n-1,bkNum));
    }
    return 0;
}