算法：60.第k个排列

解答参考：https://blog.csdn.net/lqcsp/article/details/23322951

题目链接：https://leetcode-cn.com/problems/permutation-sequence/description/

题目描述：

代码见下：

class Solution {
public:
    string getPermutation(int n, int k) {
        vector<int> num(n, );
      int perm_sum = ;
      for(size_t i = ; i < n; ++i)
      {
        num[i] = i + ;
        perm_sum *= (i + );
      }
      string ret;
      //因为数组是从0到n-1的 所以基数从 0到k-1
      --k;
      for(size_t i = ; i < n; ++i)
      {
        perm_sum = perm_sum / (n - i);
        int selected = k / perm_sum;
        ret.push_back(num[selected] + '');
        //选择一个数后重新构造剩下的数组
        for(size_t j = selected; j < n - i - ; ++j)
          num[j] = num[j + ];
        k = k % perm_sum;
      }
      return ret;
    }
};