leetcode Permutation Sequence

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本文提供了一种解决LeetCode上排列序列问题的有效方法。通过递减计数和选择算法，实现了从1到n的所有唯一排列中找到第k个排列序列。适用于n在1到9之间的场景。

Permutation Sequence 原题地址：

https://oj.leetcode.com/problems/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 (ie, for n = 3):

"123"
"132"
"213"
"231"
"312"
"321"

Given n and k, return the kth permutation sequence.

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

方法见http://blog.youkuaiyun.com/doc_sgl/article/details/12840715

public class Solution {
    public String getPermutation(int n, int k) {
		String temp = "";
		int[] nums = new int[n];
		
		int pCount = 1;
		for (int i = 0; i < n; ++i) {
			nums[i] = i + 1;
			pCount *= (i + 1);
		}
		
		k--;
		for (int i = 0; i < n; i++) {
			pCount = pCount / (n-i);
			int selected = k / pCount;
			temp += nums[selected];
			
			for (int j = selected; j < n-i-1; j++)
				nums[j] = nums[j+1];
			k = k % pCount;
		}
		
		return temp;
	}
}