Use binary search on the answer.

Using binary search check if a group of a number of people are happy. To do that take people in increasing order of their heights. Consider the ith person, his height will be i, so we should have at least a[i] persons shorter than him i.e, we should already have taken at least a[i] persons before him (number_of_people_included >= a[i]), then check if there is room for num - number_of_people_included taller people inside the group (num - number_of_people_included <= b[i]). num is the number of persons in a group.

def check(num):
	count = 0
	for i in range(n):
		if count >= a[i] and num - count <= b[i]:
			count += 1

	return count >= num

def findLargestGroup():
	r = n
	l = 0
	ans = 0

	while l <= r:
		mid = (l + r) // 2
		if check(mid):
			l = mid + 1
			ans = mid
		else:
			r = mid - 1

	return ans


for _ in range(int(input())):
	n = int(input())
	a = list(map(int, input().split()))
	b = list(map(int, input().split()))
	print(findLargestGroup())