Write an efficient algorithm that searches for a value in an m x n matrix. This matrix has the following properties:
一个 句子 指的是一个序列的单词用单个空格连接起来,且开头和结尾没有任何空格。每个单词都只包含小写或大写英文字母。
Suppose an array sorted in ascending order is rotated at some pivot unknown to you beforehand.