二分練習(C++會超時)

#include <bits/stdc++.h>
using namespace std;
int a[1000100];
int n,m,key,flag;
int check(int key)
{
    int low=0;
    int high=n-1;
    if(key>a[high])
    {
        return a[high];
    }
    if(key<a[0])
    {
        return a[0];
    }
    while(low<=high)
    {
        int mid=(low+high)/2;
        if(a[mid] == key)
        {
            return key;
        }
        else if(a[mid] >key)
        {
            high=mid-1;
        }
        else if(a[mid]<key)
        {
            low=mid+1;
        }
    }
    if(a[low]+a[high]==2*key)
    {
        flag=1;
        return low;
    }
    else
    {
        if(a[low]-key>key-a[high])
        {
            return a[high];
        }
        else
        {
            return a[low];
        }
    }

}
int main()
{
    while(scanf("%d%d",&n,&m)!=EOF)
    {
     for(int i=0;i<n;i++)
    {
        scanf("%d",&a[i]);
    }
    sort(a,a+n);
    while(m--)
    {
        flag=0;
        scanf("%d",&key);
        int t=check(key);
        if(flag==1)
        {
           printf("%d %d\n",a[t-1],a[t]);
        }
        else
        {
            printf("%d\n",t);
        }
    }
    printf("\n");

    }

    return 0;
}