leetcode[58]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.

class Solution {
public:
    int lengthOfLastWord(const char *s) {
        int res=0; 
        if(NULL==s)return res;
        int len=strlen(s);
        while(s[len-1]==' ')len--;
        for(int i=len-1;i>=0;i--)
        {
            if(s[i]!=' ')
               res++;
            else break;
        }
        return res;
    }
};