LintCode "Sort Letters by Case"

class Solution {
public:
    /** 
     * @param chars: The letters array you should sort.
     */
    bool isUpper(char c)
    {
        return c >=‘A‘ && c <= ‘Z‘;
    }
    void sortLetters(string &letters) {
        size_t len = letters.length();
        if (len < 2) return;
        
        int i = 0, j = 0, k = len - 1;
        while(i <= k)
        {
            if (isUpper(letters[j]))
            {
                swap(letters[j], letters[k--]);
            }
            else
            {
                swap(letters[i ++], letters[j]);
                j = i;
            }
        }
    }
};