AtCoderのインタラクティブ形式の問題が解けません。
http://practice.contest.atcoder.jp/tasks/practice_2

ローカルでテストしたところ成功したので、アルゴリズムは間違っていないと思います。
おそらく標準出力のflushに失敗しているのかと思うのですが、どうしたらよいでしょうか・・・
お願い致します。

# -*- coding: utf-8 -*-

#########################
#~What is this function for?~
#
#There are N balls labeled with the first N uppercase letters. The balls have pairwise distinct weights.
#You are allowed to ask at most Q queries. In each query, you can compare the weights of two balls (see Input/Output section for details).
#Sort the balls in the ascending order of their weights.
#
#This task is the Problem B of the practice contest of "AtCoder"(http://practice.contest.atcoder.jp/tasks/practice_2).
#########################

import sys
[N, Q] = list(map(int, input().split()))
Data = [chr(i) for i in range(65, 65+N)]
def QuickSort(data, start, end):
    n = end - start + 1
    pivot = int(n/2.0)
    P = data[pivot]
    light = []
    heavy = []
    for i in range(start, pivot):
        print("? "+data[i] + " " + data[pivot])
        sys.stdout.flush()
        ans = input()
        if ans == '>':
            heavy.append(data[i])
        else:
            light.append(data[i])
    for i in range(pivot+1, end+1):
        print("? "+data[pivot] + " " + data[i])
        sys.stdout.flush()
        ans = input()
        if ans == '<':
            heavy.append(data[i])
        else:
            light.append(data[i])
    l_length = len(light)
    h_length = len(heavy)

    data[start:start+l_length] = light
    data[start+l_length] = P
    data[start+l_length+1:end+1] = heavy

    if l_length >= 2:
        data = QuickSort(data, start, start+l_length-1)
    if h_length >= 2:
        data = QuickSort(data, start+l_length+1, end)
    return data

print("! " + "".join(QuickSort(Data, 0, N-1)))