60. Permutation Sequence 解题记录

题目描述：

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 (ie, for n = 3):

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

 

Given n and k, return the k^th permutation sequence.

Note: Given n will be between 1 and 9 inclusive.

解题思路：

这题借用了Next Permutation的思路，只是用k记录循环次数而已。

代码：

class Solution {
public:
    string getPermutation(int n, int k) {
        string nums = "";
        for (int i = 1; i <= n; i++)
            nums += i + '0';
        int index = 0;
        for (int j = 1; j < k; j++) {
            index = nums.size() - 1;
            while (nums[index] <= nums[index - 1] && index>1)
                index--;
            for (int i = nums.size() - 1; i >= index - 1; i--) {
                if (nums[i] > nums[index - 1]) {
                    swap(nums[index - 1], nums[i]);
                    break;
                }
            }
            reverse(nums.begin() + index, nums.end());
        }
        return nums;
    }
};