leetcode 之Permutation（七）

 void nextPermutaion(vector<int> &num)
      {
          next_permutation(num.begin(), num.end());
      }
private:
    template<typename BidiIt>
    bool next_permutation(BidiIt first, BidiIt last)
    {
        //反向，注意！
        auto rfirst = reverse_iterator<BidiIt>(last);
        auto rlast = reverse_iterator<BidIt>(first);
        auto pivot = next(rfirst);
        while (pivot != rlast && *pivot > *prev(pivot))
            pivot++;

        if (pivot == rlast)
        {
            reverse(rfist, rlast);
            return false;
        }
        //注意用法
        auto change = find_if(rfirst, pivot, bind1st(less<int>(), *pivot));
        swap(*pivot, *change);
        reverse(rfirst, pivot);

        return true;
    }

string getPermutaion(int n, int k)
      {
          string s(n, ‘0‘);
          for (int i = 0; i < n; i++)
              s[i] += i + 1;

          for (int i = 0; i < k; i++)
              next_permutation(s.begin(), s.end());

          return s;
      }