给定任意数量的不同字母，如何找到包含所有可能的两个字母序列的最短字符串？

def find_shortest_string(letters):
    # 计算可能的两个字母序列数量
    num_sequences = len(letters) * (len(letters) - 1) // 2
    
    # 构建最短字符串
    shortest_string = ""
    added_letters = set()
    
    for i in range(len(letters)):
        for j in range(i+1, len(letters)):
            # 添加字母序列到最短字符串
            sequence = letters[i] + letters[j]
            
            # 确保每个字母只出现一次
            if letters[i] not in added_letters:
                shortest_string += letters[i]
                added_letters.add(letters[i])
            
            if letters[j] not in added_letters:
                shortest_string += letters[j]
                added_letters.add(letters[j])
    
    return shortest_string

# 示例用法
letters = ['a', 'b', 'c', 'd']
shortest_string = find_shortest_string(letters)
print(shortest_string)  # 输出：'abcd'