利用好C++标准库，很好解决
字符串的排列
http://www.nowcoder.com/questionTerminal/fe6b651b66ae47d7acce78ffdd9a96c7
class Solution {
public:
vector<string> Permutation(string str) {
if (str.empty()) return {};
sort(str.begin(), str.end());
vector<string> ans;
ans.push_back(str);
while (next_permutation(str.begin(), str.end()))
ans.push_back(str);
return ans;
}
};
