字符串的排列

输入一个字符串,按字典序打印出该字符串中字符的所有排列。例如输入字符串abc,则打印出由字符a,b,c所能排列出来的所有字符串abc,acb,bac,bca,cab和cba。

算法实现如下

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashSet;

public class Solution {
	public ArrayList<String> Permutation(String str) {
		if (null == str || "".equals(str)) {
			return new ArrayList<>();
		}

		char[] ch = str.toCharArray();
		Arrays.sort(ch);
		
		// 使用HashSet避免重复，如aa的排列只有aa
		HashSet<String> result = new HashSet<>();
		permutation(ch, 0, result);
		
		// 对结果排序
		ArrayList<String> sortedResult = new ArrayList<>(result);
		Collections.sort(sortedResult);
		
		return sortedResult;
	}

	public void permutation(char[] ch, int n, HashSet<String> result) {
		if (n >= ch.clone().length) {
			result.add(new String(ch));
			return;
		}

		for (int i = 0; i <= ch.length-1; ++i) {
			char tmp = ch[i];
			ch[i] = ch[n];
			ch[n] = tmp;

			permutation(ch, n + 1, result);

			tmp = ch[i];
			ch[i] = ch[n];
			ch[n] = tmp;
		}
	}

	public static void main(String[] args) {
		Solution s = new Solution();
		ArrayList<String> result= s.Permutation("abcdef");
		
		System.out.println(result);

	}
}