LEETCODE60——第K个排列

 class Solution {
 public:
     string getPermutation(int n, int k) {
         string ans(n, '');
         vector<bool> flag(n, false);
         int count = ;
         for (int i = ; i < n; i++)
             count *= i;
         int m;
         int m_f;
         int index = ;
         k = k - ;
         while (index <n-)
         {
             m = k / count;
             m_f = ;
             for (int i = ; i <= m; i++, m_f++)
             {
                 while (flag[m_f])
                     m_f++;
             }
             m_f--;
             flag[m_f] = true;
             ans[index] = m_f + + '';
             index++;
             k = k % count;
             count = count / (n - index );
         }
         int i = ;
         while (flag[i])
             i++;
         ans[n - ] = i +  + '';
         return ans;

     }
 };

总结：几乎是用找规律算出来的，再次写，未必能流畅写下来。

1. "123"
2. "132"
3. "213"
4. "231"
5. "312"
6. "321"