多变量递归-全排列问题

问题描述：给定一个没有重复数字的序列，返回其所有可能的全排列。

/**
*Given a sequence with no duplicate numbers, return all its possible permutations.
*given an array int arr[] = {1,2,3}
*output 
*[1,2,3]
*[1,3,2]
*[2,1,3]
*[2,3,1]
*[3,1,2]
*[3,2,1]
*/
import java.util.*;public class Permutations {public List<List<Integer>> permute(int[] nums) {List<List<Integer>> res = new ArrayList<>();backtrack(nums, new ArrayList<>(), new boolean[nums.length], res);return res;}private void backtrack(int[] nums, List<Integer> path, boolean[] used, List<List<Integer>> res) {if (path.size() == nums.length) {res.add(new ArrayList<>(path));return;}for (int i = 0; i < nums.length; i++) {if (used[i]) continue;used[i] = true;path.add(nums[i]);backtrack(nums, path, used, res);used[i] = false;path.remove(path.size() - 1);}}public static void main(String[] args){int[] arr = {1,2,3,4};List<List<Integer>> res = permute(arr);for(List<Integer> aList : res){System.out.println(aList.toString());}}
}