47-Permutations2.cc

class Solution
{
  public:
    vector<vector<int>> permuteUnique(vector<int> &num)
    {
        sort(num.begin(), num.end());
        unordered_map<int, int> count_map; // 记录每个元素的出现次数
        for_each(num.begin(), num.end(), [&count_map](int e) {
            if (count_map.find(e) != count_map.end())
                count_map[e]++;
            else
                count_map[e] = 1;
        });
        // 将map里的pair拷贝到一个vector中
        vector<pair<int, int>> elems;
        for_each(count_map.begin(), count_map.end(),
                 [&elems](const pair<int, int> &e) {
                     elems.push_back(e);
                 });
        vector<vector<int>> result; // 最终结果
        vector<int> p;              // 中间结果
        n = num.size();
        permute(elems.begin(), elems.end(), p, result);
        return result;
    }

  private:
    size_t n;
    typedef vector<pair<int, int>>::const_iterator Iter;
    void permute(Iter first, Iter last, vector<int> &p,
                 vector<vector<int>> &result)
    {
        if (n == p.size())
        { // 收敛条件
            result.push_back(p);
        }
        // 扩展状态
        for (auto i = first; i != last; i++)
        {
            int count = 0; // 统计字符 *i 在p中出现过多少次
            for (auto j = p.begin(); j != p.end(); j++)
            {
                if (i->first == *j)
                {
                    count++;
                }
            }
            if (count < i->second)
            {
                p.push_back(i->first);
                permute(first, last, p, result);
                p.pop_back(); // 撤销动作，返回上一层
            }
        }
    }
};