Length of Last Word

题目：

Given a string s consists of upper/lower-case alphabets and empty space characters ' ', return the length of last word in the string.

If the last word does not exist, return 0.

Note: A word is defined as a character sequence consists of non-space characters only.

For example, 
Given s = "Hello World",
return 5.

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题意：一个字符串，返回最后一个单词的长度，每个单词之间均有一个' '。

理解：这题比较简单，直接从字符串的从后往前遍历遇到第一个' '返回长度就可以了，注意边界情况