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(string s) {
        int n = s.size();
        int start, end;
        for ( start = n-1; start >= 0; start-- ) if ( s[start] != ' ' ) break;
        if ( start == -1 ) return 0;
        for ( end = start; end >= 0; end-- ) if ( s[end] == ' ' ) break;
        return start - end;
    }
};