题目：

题解：

class Solution {
public:string getPermutation(int n, int k) {vector<int> factorial(n);factorial[0] = 1;for (int i = 1; i < n; ++i) {factorial[i] = factorial[i - 1] * i;}--k;string ans;vector<int> valid(n + 1, 1);for (int i = 1; i <= n; ++i) {int order = k / factorial[n - i] + 1;for (int j = 1; j <= n; ++j) {order -= valid[j];if (!order) {ans += (j + '0');valid[j] = 0;break;}}k %= factorial[n - i];}   return ans;     }
};