LeetCode:Length of Last Word

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.

这个简单的题目花了不少时间，各种出错。后来发现“a ”通过不了，最后在for循环之前加了一个while先去掉最后的空格 通过。

class Solution {public:    int lengthOfLastWord(string s) {        int num=0;        if (s.length()) {            int r = s.length()-1;            while (s[r]==' ') {                r--;            }            for ( ; r>=0; r--) {                if(s[r]==' '){                    break;                }                num++;            }        }        return num;            }};