Permutations LeetCode

 Given an array nums of distinct integers, return all the possible permutations. You can return the answer in any order.

 

Example 1:

Input: nums = [1,2,3]

Output: [[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]

Example 2:

Input: nums = [0,1]

Output: [[0,1],[1,0]]

Example 3:

Input: nums = [1]

Output: [[1]]

import java.util.*;

class Solution {

    public List<List<Integer>> permute(int[] nums) {

        List<List<Integer>> result = new ArrayList<>(); // to store all permutations

        boolean[] used = new boolean[nums.length]; // track used elements

        List<Integer> path = new ArrayList<>(); // current permutation path

        backtrack(nums, path, used, result);

        return result;

    }

    private void backtrack(int[] nums, List<Integer> path, boolean[] used, List<List<Integer>> result) {

        if (path.size() == nums.length) {

            result.add(new ArrayList<>(path)); // important: create new list and add

            return;

        }

        for (int i = 0; i < nums.length; i++) {

            if (used[i]) continue; // skip if already used

            // Choose

            path.add(nums[i]);

            used[i] = true;

            // Explore

            backtrack(nums, path, used, result);

            // Unchoose (Backtrack)

            path.remove(path.size() - 1);

            used[i] = false;

        }

    }

}