Given a string and a positive integer d. Some characters may be repeated in the given string. Rearrange characters of the given string such that the same characters become d distance away from each other. Note that there can be many possible rearrangements, the output should be one of the possible rearrangements. If no such arrangement is possible, that should also be reported.
Expected time complexity is O(n) where n is length of input string.

Examples:
Input:? "abb", d = 2
Output: "bab"

Input:? "aacbbc", d = 3
Output: "abcabc"

Input: "geeksforgeeks", d = 3
Output: egkegkesfesor

Input:? "aaa",? d = 2
Output: Cannot be rearranged
摘自：http://www.geeksforgeeks.org/rearrange-a-string-so-that-all-same-characters-become-at-least-d-distance-away/

這題就是貪心算法。對于字符串處理來講，如果不care最終的符號序，就要以考慮用統計重排的方法。

1. 統計所有字符的出現頻率；

2. 按出現頻率從高到低優先填好所有字符；

如果全部都是ASCII碼，共出現m個不同字符，第2步，可以：

1. 用最大堆來做，也可以直接排序，時間復雜度都是O(n+mlgm)，空間復雜度就是O(m)。因為m<256<<n，所以最終復雜度也可以說是O(n)。

2. 也可以直接用一個vector<list<char> >記錄每個頻率下的字符來做。用vector<list<char> >就是O(2n)的時間復雜度了。空間復雜度高點，O(n)。

?

Rearrange a string so that all same characters become d distance away