Given a string, you need to reverse the order of characters in each word within a sentence while still preserving whitespace and initial word order.
Input:
"Let's take LeetCode contest"
Output:
"s'teL ekat edoCteeL tsetnoc"
Note: In the string, each word is separated by single space and there will not be any extra space in the string.
思路：用空格拆分字符串，反转每一个单词，再 join 成一个新的字符串
