Lower_bound

int lp(ll *arr , ll num , ll n){
    if(num > arr[n-1])return n;
    if(num <= arr[0])return 0;
    int l = 0 , r = n , mid;
    while(l < r){
      mid = l + (r-l)/2;
     if((mid!=0 && (arr[mid-1] < num && arr[mid] >= num)))return mid;
      if(arr[mid] >= num)
      r = mid - 1;
      else
      l = mid + 1;
    }
    return l;
}