#49 Sort Letters by Case

题目描述：

Given a string which contains only letters. Sort it by lower case first and upper case second.

 Notice

It's NOT necessary to keep the original order of lower-case letters and upper case letters.

Have you met this question in a real interview? 

Yes

Example

For "abAcD", a reasonable answer is "acbAD"

Challenge 

Do it in one-pass and in-place.

题目思路：

就用two pointers的方法，用l指针始终指向lower case letters，用r指针始终指向upper case letters。当l<r并且l指向了upper case，r指向了lower case的时候，就swap两个char。

Mycode（AC = 16ms）：

class Solution {public:    /**      * @param chars: The letters array you should sort.     */    void sortLetters(string &letters) {        // write your code here        int l = 0, r = letters.length() - 1;        while (l < r) {            while (l < r && lowercase(letters[l])) {                l++;            }            while (l < r && !lowercase(letters[r])) {                r--;            }                        if (l >= r) break;            swap(letters, l, r);        }    }        bool lowercase(char ch) {        return ch >= 'a' && ch <= 'z';    }        void swap(string &letters, int i, int j) {        char ch = letters[i];        letters[i] = letters[j];        letters[j] = ch;    }};