二分

int l=0,r=maxn-1;
int ans=-1;
while(l<=r)
{
    int mid=(l+r)/2;
    if(check(mid))
    {
        ans=mid;
        r=mid-1;
    }
    else
    {
        l=mid+1;
    }
}
if(ans==-1)
    printf("Not Find!\n");
else
    printf("%d\n",ans);