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.

解题思路：

遍历字符串，设置整形变量n，记录当前遍历到的无空格子串的长度，如果子串后遇到空格，且空格后再次遇到新的子串，则更新n，否则返回n；

附录：

C语言中的char* char const char 和C++ string的关系

【Leetcode】【Easy】Length of Last Word