本地运行正确洛谷显示read (ASCII 0)

#include <iostream>
#include <cmath>

int n;
char btree[1500], tmp;
int len, arrlen;

void dfs(int i)	//递归输出
{
	if (i > arrlen) {
		return ;
	}
	dfs(2 * i);
	dfs(2 * i + 1);
	std::cout << btree[i];
}

int main(void)
{
	std::cin >> n;
	arrlen = std::pow(2, n + 1);
	len = std::pow(2, n);
	for (int i = len; i < 2 * len; i++) {	//建立叶子结点
		std::cin >> tmp;
		if (tmp == '0') {
			btree[i] = 'B';
		} else if (tmp == '1') {
			btree[i] = 'I';
		}
	}
	for (int i = n - 1; i >= 0; i--) {	//建立其他结点
		len = std::pow(2, i);
		for (int j = len; j < 2 * len; j++) {
			if (btree[2 * j] == 'I' && btree[2 * j + 1] == 'I') {
				btree[j] = 'I';
			} else if (btree[2 * j] == 'B' && btree[2 * j + 1] == 'B') {
				btree[j] = 'B';
			} else {
				btree[j] = 'F';
			}
		}
	}
	dfs(1);
	return 0;
}