LeetCode-60. Permutation Sequence

The set ​​[1,2,3,...,n]​​ contains a total of n! unique permutations.

By listing and labeling all of the permutations in order, we get the following sequence for n = 3:

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

Given n and k, return the kth permutation sequence.

Note:

• Givennwill be between 1 and 9 inclusive.
• Givenkwill be between 1 andn! inclusive.

Example 1:

Input: n = 3, k = 3
Output: "213"

Example 2:

Input: n = 4, k = 9
Output: "2314"

题解：

每次获取当前的首数字，直到最后一个数字。

class Solution {
public:
    string getPermutation(int n, int k) {
       vector<int> total(n + 1, 1);
       vector<char> num;
       string res;
       for (int i = 1; i <= n; i++) {
           num.push_back(i + 48);
       }
       for (int i = 2; i <= n; i++) {
           total[i] = total[i - 1] * i;
       }
       for (int i = n; i > 0; i--) {
           int temp = (k - 1) / total[i - 1];
           res += num[temp];
           num.erase(num.begin() + temp);
           k -= temp * total[i - 1];
       }
       return res;
    }
};