Processing math: 100%
(Analysis by Nick Wu)

We start by defining a winning position and a losing position. If the current player wins given optimal play on both sides, the position is a winning position, otherwise the position is a losing position.

For positions that are not immediately winning or losing positions, we can make the following two observations:

In the context of this problem, the pile having zero stones is a losing position, the pile having 1,2,,9 stones is a winning position, and the pile having ten stones is a losing position.

Subtask 1: S<100.

We can naively check this process and compute for all S in increasing order if they are winning positions or losing positions.

T = int(input())
 
def is_pal(j):
	s = str(j)
	return s == "".join(reversed(s))
 
for _ in range(T):
	S = int(input())
	win = [False] * (S+1)
	for i in range(S+1):
		for j in range(1, i+1):
			if is_pal(j) and not win[i-j]:
				win[i] = True
	print("B" if win[S] else "E")

Subtask 2: S<106.

This subtask rewarded people for experimenting with moves and only trying moves that involved taking at most 9 stones.

T = int(input())

S = 10**6
win = [False] * (S+1)
for i in range(S+1):
	for j in range(1, min(i+1, 10)):
		if not win[i-j]:
			win[i] = True
 
for _ in range(T):
	S = int(input())
	print("B" if win[S] else "E")

Subtask 3: S<109.

If we look at the winning position and losing positions, we can see that a position is winning if and only if S is not divisible by 10. The proof of this follows directly from observing that taking anywhere from 1 to 9 stones is possible, but it is never possible to remove a multiple of ten. Therefore, we just naively check if the integer is divisible by 10.

Full Credit:

To check if a very large integer is a multiple of 10, it suffices to check if the last digit of the number is 0.

Benjamin Qi's code:

T = int(input())
for _ in range(T):
	start = input()
	print('E' if start[-1] == '0' else 'B')