写出宝石排列问题的C++代码，并给出测试用例

宝石排列问题可以用回溯算法来解决。以下是C++代码：

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

void backtracking(vector<char>& candidates, vector<char>& path, vector<vector<char>>& result) {
    if (path.size() == candidates.size()) {
        result.push_back(path);
        return;
    }
    for (int i = 0; i < candidates.size(); i++) {
        if (i > 0 && candidates[i] == candidates[i-1] && find(path.begin(), path.end(), candidates[i-1]) == path.end()) {
            continue;
        }
        if (find(path.begin(), path.end(), candidates[i]) != path.end()) {
            continue;
        }
        path.push_back(candidates[i]);
        backtracking(candidates, path, result);
        path.pop_back();
    }
}

vector<vector<char>> getGemstones(string s) {
    vector<char> candidates(s.begin(), s.end());
    sort(candidates.begin(), candidates.end());
    vector<vector<char>> result;
    vector<char> path;
    backtracking(candidates, path, result);
    return result;
}

int main() {
    string s = "ABCD";
    vector<vector<char>> result = getGemstones(s);
    for (auto item : result) {
        for (auto c : item) {
            cout << c << " ";
        }
        cout << endl;
    }
    return 0;
}

测试用例：

输入："ABCD"

输出：

A B C D

A B D C

A C B D

A C D B

A D B C

A D C B

B A C D

B A D C

B C A D

B C D A

B D A C

B D C A

C A B D

C A D B

C B A D

C B D A

C D A B

C D B A

D A B C

D A C B

D B A C

D B C A

D C A B

D C B A