第三题 python 解法 https://github.com/981377660LMT/algorithm-study/blob/master/21_%E4%BD%8D%E8%BF%90%E7%AE%97/%E4%BA%8C%E8%BF%9B%E5%88%B6%E6%9E%9A%E4%B8%BE%E4%B8%8E%E4%B8%89%E8%BF%9B%E5%88%B6%E6%9E%9A%E4%B8%BE/%E4%B8%89%E8%BF%9B%E5%88%B6%E6%9E%9A%E4%B8%BE/%E8%B6%B3%E7%90%83%E6%AF%94%E8%B5%9B%E5%BE%97%E5%88%86.py